2500 point spell crits with an 18% chance (sans raid gear) apparently isnt enough. Now we need PREs to make them even more powerful.
Wizards / Sorcs can already solo many zones that melee find impossible or at least MUCH harder to solo.
You could package up any of the fire / ice / acid / lightning / force etc...spec of casters into a "prestige" if you wanted to. Also, Necromancer seems to be a viable build with higher DC death spells and negative energy effects.
90% of the casters I see are still fire / ice though.
Alot of the wizard prestige classes had requirements that are not in DDO, like knowledge skills etc. and are harder to adapt to a video game than melee classes. Also - alot of the PnP prestige classes use spells that are not in the video game DDO. Casters have VERY shortened spell lists in DDO from their PnP counterparts. In order to adapt these prestige classes they would have to put more spells into the game. Its still possible but it will take some time to do.