Unless you go for Ur-Priest route which gives 6th level spells. Yuan-ti abomination also has 9 HD which count as manifester levels, so it can enter metamind PrC instant but features mean nothing to it as it lacks psionic powers which use power point (if we use expanded psionic variant they get psionic subtype and bunch psionic powers which mimic most what it had in normal DMG but no power points).<br/><br/>There are options though; entering half-fiend allows give you more skill points than monstrous humanoid from your racial HD and get spell-alike abilities and boost to stats and resistances (and quite slow flight speed). It makes you CR 10 so got 3 more CR to go for class levels, or take half-dragon template (+3 CR) top of that to get truly monstrous amount of stat boost (+4 natural armor) and fly speed of 60'ft and also breath attack. As there is some art of winged female lamias around ;).<br/><br/>However sadly they cannot enter phrenic creature which would give tons more psionic powers as they are already considered psionic creature.<br/><br/>Now, as GM I would make house ruling the manifester levels as yuan-ti work same as raksha outsider HD levels work for sorcerer; they stack with class levels for purposes to determine what powers they get and what not.<br/><br/>Now big question is what they are psionically speaking? If they would be psions they would have 19 different powers in 9th level across several levels. However they got only 13 powers; Aversion, Chameleon, Psionic Daze, Detect Hostile Intent, Entangling Ectoplasm, Exhalation of Black Dragon, Body Purification, Psionic Charm, Concealing Amorpha, Deeper Darkness/Control Light&Darkness (only create darkness), Mind Thrust, Psionic Suggestion, Baleful Polymorph<br/><br/>All above in various forms of augmentation and differences. Their not wilder nor quite psions either. So to make it more viable as a PC (and easier determine what PrC's it can enter) I treat the 9 HD as levels as psion (with no designated discipline). So you get power points and 19 different powers up to 5th level (although Exhalation of Black Dragon is 6th level power but it's psi-alike ability).<br/><br/>Now as 9th level psion you should have now 6 level in any psionic PrC; Thrallherd could work (kind cultist leader/charmer) or enter Metamind so you can truly dominate in psionic power point economy (and get up to 6th level psionic powers and use psi-crystal as power point container and freely use lower level psionic powers minus augmentations).<br/><br/>Naturally you can pick expanded power to get any powers from wanted disciplines like psionic charm from telepath (so minor drawback compared to normal humanoid who picked psion class since first level to get access to discipline power lists while your stuck with generic list). But Thrallherd gives psionic charm and dominate for free anyways. It needs mindlink though for PrC qualification and it's first level telepath power.<br/><br/>Or you can enter other psionic PrC's instead.