Seems interesting. But you were wrong about it being not confusing :P. Some clarification required: First, do the 2 monsters have to have levels that add to that of the summoned monster or both have the same level, like with XYZ? Second: well, this one's probably easier to illustrate.
So, I have my A monster, lets say it's this:
"Generic A monster" LIGHT
Level 4
2000 ATK 1000 DEF
Effect: "Once only, you may pay 1000 life points to destroy 1 monster on the field."
And I have my B monster:
"Generic B monster" EARTH
Level 4
1800 ATK 1200 DEF
Effect: "By paying 500 life points, destroy 1 spell or trap card on the field"
And I phase summon my Phase monster
"Generic Phase Monster" DARK
Level/Rank 4
? ATK ? DEF
Effect: ---
I summon it in A phase, so it's then got only B as a material.
"Generic Phase Monster" DARK
Level/Rank 4
2000 ATK 1000 DEF
Effect: "Once only, you may pay 1000 life points to destroy 1 monster on the field."
I activate it's effect and destroy my opponent's Blue Eyes Ultimate Dragon.
I then have to have it switch to B form, and it has no materials at all
"Generic Phase Monster" DARK
Level/Rank 4
1800 ATK 1200 DEF
Effect: "By paying 500 life points, destroy 1 spell or trap card on the field"
Now, heres where the problem arises. Now that I've activated the effect, does it remain as a 1800 ATK 1200 DEF monster, or default to 0/0?
Also, does it keep it's B effect to use again or does it lose that but not the ATK and DEF boost? Also, is the ATK and DEF classed as a boost or does it class as Original ATK/DEF?
I now Overphase it, choosing A
It becomes
"Generic Phase Monster" DARK
Level/Rank 4
2000 ATK 1000 DEF
Effect: "Once only, you may pay 1000 life points to destroy 1 monster on the field."
again.
I can use it's effect once more right?
Sorry for my effect writing, i've been making a lot of MTG cards recently so my terminology may be incorrect.
Also, do the phase monsters go in the extra or main deck?
The problem I see with Phase summoning is that it stops the player from having that one awesome signature card, like Yusei's Stardust Dragon, because no matter how awesome the phase summoned monster, its still only as powerful as a level 4 or 7 or whatever.
For reference, I was thinking of implementing a variation of ritual summoning called "Manifest summoning". The monsters summoned are a new kind of monster, like Synchro and XYZ, called Aetherial. Aetherial monsters are shuffled into the normal deck. you summon them by sending 1 spell card from your hand or field to the grave, as well as monsters whose total levels are equal to or greater than the Aetherial monster's level. The aetherial monsters have their own effects, ATK, DEF etc, but also gain the effect of the spell card sent. If the spell's effect is continuous, like the field spell "Yami" then the effect will be permanent, so all spellcasters and fiends gain 200 ATk and DEF. If the spell's effect is temporary, such as Spell Shattering Arrow, then it becomes a "Once per turn" effect. IE for spell shattering arrow it would be "Once per turn, you may destroy all spell cards your opponent controls and inflict 500 damage to their life points for each". Difficulty arises when using things like Swords of Revealing Light, which destroy themselves after 3 turns, as it means that the Aetherial monster would be destroyed after the 3 turns are up. The catch to these otherwise OP monsters is that you may only control 1 Aetherial monster at a time, so no one can spam stuff.