Now, I have a peculiar idea for a pure three-stage Psychic type, a la Abra and Ralts...
It would chronicle the development of the various language facilities of the brain. It would start out being able to hear but not be able to make sense of anything it hear, and it would not be able to manipulate the articulations of its tongue and convey thoughts through speech. Upon its first evolution, it could interpret and make sense of language, but it would still not be able to express its own thoughts through speech. Then, come the next evolution, it would have fully capable linguistic abilities and able to understand and express throughts in myriad languages. It would have the uncanny ability to be able to learn a language over the course of a day or two (for some Pokedex fodder).
Glottauri -> Glottgona -> Glottlang
As for its merit as a battler, obviously it would get your Psychic moves. In addition, it could learn a smattering of additional attacks, especially those related to sound and the development of language. Mimic, Water Pulse, Supersonic, Hyper Voice, etc. However, it would also have a couple language-related moves, such as Translate, which would allow it to adopt the type of move that was last used against it (Psychic means single-typed again), Mimic to show how languages are learnt by children by listening to those around them, etc.
But of course it would get your Psychic staples of the elemental punches. However, I'm thinking that it should either of Thunderbolt or Ice Beam or some pernicious, damaging Dark move. Either way, I want it to get one of those.
Stats are high Special Attack and Speed, nothing to see here.