You're right - but I can just throw a Ghostbreaker level in there for filler, like Eldritch Knight levels in an arcane build.
And you're right about Illithid Slayer, of course. How does Monk 1/Psion 4/Anarchic Initiate 1/Ghostbreaker 1/Illithid Slayer 3/Legacy Champion 6/Illithid Slayer 2(9)/Ghostbreaker 2(3) sound? I still get a lot of features from the PrCs, but I also get the free Empower and the Monk abilities, which are also useful in conjunction with Metamorphosis.
Monk 1 = +0 BAB
Psion 4 = +2 BAB
Anarchic initiate = +0 BAB
Ghostbreaker = +1 BABBAB =/= +4.
I'd skip monk entirely and grab a monk's belt and
metamorphosis, perhaps burning a feat on Improved Unarmed Strike. If you
must burn a manifester level, try a 1-level dip into lion totem barbarian for pounce. Or just squeeze in a level of ghostbreaker instead of anarchic initiate, as a non-blaster gets very little out of AI.
Finishing with BAB +16/ML 17.
BAB isn't nearly as important to you as it would normally be, assuming you rely on natural attacks. For you, it's attack bonus that's important, and hitting REALLY DAMNED HARD.
Of course, if you go for Power Attack it's a bit more important, but the difference between +15 and +17 BAB is fairly small.
Legacy Champion could be cut - the class features may be amazing, but medium BAB hurts.
Why? The class features are better than the BAB, since you don't need it as much as, say, a fighter. Stick with forms that add natural attacks to your normal attack routine and you actually come out ahead in the long run.
It's still 2-level minimum, unless I want another Psion level to net Telepathy, and then I may just go Psion 6 since it'll progress BAB. Something to consider.
[EDIT] Dropping 4 Legacy levels and replacing them with Illithid Slayer bumps the BAB up to +17, but I lose some neat stuff.
Don't lose the neat stuff. Trust me. Proper power and feat selection > BAB for a manifester...even a gish.
Let's say that at level 20 you have a BAB of +14. Normally this wouldn't be that great. It grants you an attack routine of +14/+9/+4.
But you've also taken Metamorphosis and Illithid Heritage + Illithid Grapple x4. You use girallon as your current combat form. You wield a quarterstaff 2-handed (since it's a simple weapon) and get an attack routine of +14 quarterstaff/+9 bite/+9 claw/+9 claw/+9 quarterstaff/+9 tentacle/+9 tentacle/+9tentacle/+9 tentacle/+4 quarterstaff.
And you get even more if you share Metamorphosis on your psicrystal as well.
The nifty class features are MUCH better than BAB, especially since you can make touch attacks with feats and spells and such, can gain a ton of natural attacks to boost your routine, and can gain damage and accuracy in other ways as well.
BAB is overrated for someone who, as Sunic_flames puts it, is a REAL character.
Eh... Sorry for the mistake on your name.
Just don't let it happen again.