I think it's the plot(though I' terrible with that),
I don't think plot is all too important at all, really. An interesting idea for an RP might attract some players, but, an idea that's
too outlandish might actually scare players away more than anything else. People like familiarity. A concept similar to something that people have already seen before makes it a lot easier for people to get into it. It's the reason why high school settings and fandom RP's are so popular. People know what to expect -- and it isn't difficult to wrap their heads around.
So, don't concern yourself with creating an "original" plot. Trying too hard to be "original" might actually do you more harm than good. :P Just create an RP that you think would be fun to play. It doesn't matter if it's a common RP idea. If anything, that just makes it more likely that you'll have a stable player base.
Oh! But one thing I should mention: make sure your idea actually works well
as a roleplay, not just as an idea by itself. You could come up with the most fascinating setup ever, but, if you only think about the plot and worldbuilding without considering the mechanics of it, you might find your brilliant idea crashing and burning before too long. For example, I once had an idea for a modern fantasy RP about stone dragon figurines that were actually
real dragons sealed into a tiny stone form by an ancient curse. And specific people who found those figurines could bring the dragons back to life, but the change would occur in stages (so, they wouldn't immediately become full-size dragons -- instead, they'd be rather tiny for a while). Sound like an interesting idea? I thought so. In fact, I liked this idea so much that I actually tried it out
twice (though the two incarnations occurred several years apart). Trouble is,
both versions of that RP crashed before long. And while the 2nd one did last slightly longer after having learned from the first one, they both suffered from being approached in the same flawed way.
The problem with it was, I was so wrapped up in what a fun idea I thought it was that I didn't at all think about how it would play out as an RP. In the first RP, I wound up playing
all the dragons (and there were about 8 of them, I think) in order to decide when the dragon would transition from stage to stage in the re-awakening process depending on its interactions with the human, and so that I could reveal information about them at my own pace. The most obvious problem that this suffered from was that I could
not keep up with playing that many characters. @_@ Additionally, the players didn't have nearly as interesting of a role. With each of them playing one of the humans that found a dragon, their actions were primarily limited to "holy shit! That toy dragon just
talked!", as well as asking questions about it and all once they'd started to accept the reality of what was going on. Problem was, the human characters hardly at all interacted with each other -- they pretty much only interacted with their dragons. Which sort of makes sense in the context of the story, but it just
does not work as a good setup for an RP.
In the 2nd version of this RP, I tried to remedy the problems of the first by making the dragons into player characters, as well, but that came with its own slew of problems. Firstly, this essentially meant that each human character was stuck with a specific dragon character, and if the player behind either of those characters dropped out, then their partner character was stuck. I'm going to make this clear right now:
you cannot assume that your RP won't suffer from drop-outs. I tried to make it very clear that this RP needed dedicated players in order to work properly, but, drop-outs happen. You can't prevent them. Even if all your players
want to be dedicated, life happens, and people lose interest. As you might imagine, the dropping out of players in this RP made it near-impossible for other areas of the RP to continue. And, like with the first one, there was still the problem of human characters not interacting enough. I think we
tried to make the human characters interact, but, you know, with all of them trying to keep their dragons a secret from each other... yeah. It just didn't work out.
See what I'm trying to say about keeping mechanics in mind? My idea about the stone dragons could've worked for, say, a novel or something -- but as an RP, it just had way too many problems to make it work, at least in the way(s) I tried to present it.
Think about how the idea for your RP is actually going to play out at a mechanical level. And by "mechanics", I don't mean tabletop-style stats (not unless you're actually running that kind of RP). I just mean in the way of how all your players are going to interact with each other. Creating a stable foundation that encourages activity instead of stifling it is a
huge factor in whether or not your RP will last.
For a more
positive example of plot affecting this sort of thing, let's use my current RP, Fandomstuck, as an example. It's probably one of the most successful I've had in a while, and a lot of that was due to aspects of it creating that "stable foundation" I mentioned -- but, in a lot of ways I wasn't even thinking about when I created it. OwO You see, Fandomstuck is sort of like a crossover fandom RP, except that it focuses on personifications of entire fandoms instead of specific canon characters (so, there are characters that represent the entirety of things like Doctor Who, Homestuck, etc). What's fascinating about Fandomstuck is that it wound up taking all of the usual flaws present in a mega-crossover RP and completely resolving them. For example, the way mega-crossover RP's usually work is that you have a bunch of canon characters from different universes all coming together in one universe for whatever reason. Then, you have to spend a
ton of time just having those characters get used to the situation they're in and the characters that surround them. What if one character present is a magic-user, while another character is from a universe where magic doesn't exist? How are they supposed to react to that? And what if the setup of the RP makes it clear that all of them are just considered fictional characters in "this" universe? It takes a lot of time to get over all that. And then, once all of that
is taken care of, the players all have to wait for the GM to get some plot started that involves all of them -- or else the RP will stagnate as the characters run out of idle chit-chat.
Fandomstuck remedies so many of these problems. First of all, the fandom entities all know that they're based off of various TV shows, video games, etc (and are rather comfortable with that reality), so, no need to worry about any identity crises. Secondly, they know that the
other fandoms are based off of canons with very different rules from their own, so you can have sci-fi and fantasy fandoms interacting with each other without being freaked out by each other's magic/technology/etc. Sooo
that's not a thing we need to worry about.
And, lastly, Fandomstuck, or at least the version of it that I'm running (as mine was partially inspired by a short-lived Fandomstuck RP that a friend of mine ran), also solves the problem of waiting for a GM to run some plot and running the risk of stagnating if they don't. Because you see, one of the things that I really disliked about my friend's Fandomstuck RP (and one of the reasons why it fell apart so quickly, imo) is that all the fandoms could only sort of hang around and make idle chit-chat while they waited for the plot to get going. The setting didn't really allow them to do much of anything else. So, in mine, I wanted to give the players a lot more freedom. It's much more sandboxy, and allows pretty much anyone to start up their own side-plot using things from their respective canons. And... it works! The RP is pretty much in a constant state of players having things to do, all because of the side-plots. Most of these side-plots I have some hand in coordinating once they reach the point of affecting pretty much every character, but they usually start out at the level of players just doing whatever and letting fandoms bring about all sorts of shenanigans. So far we've had an outbreak of Trickster Mode, an Indominus Rex on the loose, and we're currently about to deal with a Dalek invasion. The RP pretty much naturally splits itself up into arcs like this without me even needing to do much to orchestrate all of it. XD There's really never a dull moment. Oh! And drop-outs aren't an issue, either, as it's easy to remove any character from the spotlight when the need arises.
So, yeah, in short, just make sure you think through the mechanics of an idea before you decide it would make a good RP. Oh, and make sure that players always have
something to do. That doesn't necessarily mean giving them a strict progression of events to play through. Sometimes it can be more sandboxy like my Fandomstuck,
but only if a sandbox would work well for that concept. You have to think it through and decide which is more appropriate for your given RP. Sometimes an RP needs more strict GM control of the plot, other times it can be more free and player-run. Just make sure you go with whichever choice your RP needs. More GM control than needed will stagnate creativity and make players feel railroaded. Not enough GM control will lead to a sense of "soooo what now?" as everyone's characters run out of idle chit-chat and players get bored. It's a balance that's hard to pull off, but, it's important to try.
So really, I don't think the
concept of a plot matters it all. It's all about how you
structure your RP to match that plot. :P
And, yeah, as others said, a dedicated GM and dedicated players are also hugely important. But even if a GM is
dedicated, they can't do much to save an RP if they don't know how to run it properly. And, as stated before, you can
never guarantee dedicated players, but you can do your best to structure the RP in a way that keeps them interested, and that won't be too difficult to deal with when drop-outs inevitably happen. It's a strategic structure to RP's that I think
really keeps them working well.
That's what I'd focus on, if I were you.