I want to be a specialist wizard! Right now, in DDO, even after taking the appropriate Spell Focus feats, you're really only a generalist wizard. In PnP, a wizard could sacrifice the use of two schools to specialize in one. This translated into 1 bonus spell per level from your chosen school.
The easiest way to implement this in DDO is either 1) a discount on spells from the chosen school (10%-20% would be sufficient) or 2) a bonus to caster level when casting spells of that level (+1 or +2 is enough, the largest benefit of this is for spell penetration checks.).
However, that's not very sexy, so I thought up some other ideas based on each school:
For Charm and Dominate spells, your caster level is added to the time between saves.
Nearby monsters become shaken for 1 min when you cast a necromancy spell. (No save)
Enervation and Energy Drain deal 1 additional negative level.
Your damage spells deal additional untyped damage equal to your caster level (added before feats/enhancements).
Your summoned creatures receive an additional +2 to all stats (that stacks with Augment Summoning).
Your Extended beneficial Transmutaion spells cost 2 SP less than normal.
You gain spell resistance 5 that stacks with all other sources of spell resistance.
You gain energy resistance (all types) 3 that stacks with all other sources of energy resistance.
You gain a +1 insight bonus to saves.
Nearby monsters become disoriented for 1 min (-2 to attack rolls) when you cast an Illusion spell. (No save)
I tried to keep the bonuses small enough to not need to justify giving up an entire school of magic, let alone two. There really aren't enough spells in the game to justify that. Let me know what you think.