The following suggestion would improve the game, but is of low priority compared to other potential changes. Casters in D&D and DDO have the problem that their spells per day are pretty limited, even if they're only using weak spells. In D&D this is solved with "Reserve Feats" from Complete Mage, and it's possible to add something similar to DDO as well; I'll call them "Spell Reserve" abilities, to emphasize that they relate to spellcasting and that they're not actually feats.
Benefits
Spell-reserve abilities would improve verisimilitude, engagement, and balance.
1. Verisimilitude: because a supposedly powerful sorcerer just doesn't feel convincing if he can't shoot off a little magic because he might need a big spell later in the day. That's like a kung-fu master who's afraid to walk around in case he needs his feet for kickboxing later.
2. Engagement: It's fun to be engaged with a game, which means you're doing something. DDO values engagement more highly than some other games that include /follow /assist /autoattack. To wait around because you can't do anything helpful is not engagement, and to stand around watching for a slight chance to be helpful is weak engagement. Too often when there's a DPS (or con damage) beatdown, spellcasters have no good way to contribute: casting a spell wastes points that might be needed later, and attacking with a wand or weapon has little effect, and interferes with your items so that in the event you do have to cast a spell in reaction to a sudden emergency, you won't do it as well.
Frequently in a big raid you'll see casters fidgeting watching the team fight some large devil, and not doing anything to help so they can save mana for gnolls or bats later. Only if they have nearly perfect prediction of what will happen next are they liberated to throw in some DPS.
3. Balance: Spellcasters have the best hp DPS, especially if the monsters group up, but they have no way to sustain it over time. Warrior classes may lose DPS as time goes on and they lose boosts and rages, but that's nothing like how it works for a mage. Quest designers' ability to build winnable challenges is hampered because the kinds of DPS classes bring such divergent capabilities. If casters had a way to inflict long-term DPS without expending real mana, their damage profile would come closer to other characters. (And then if you went ahead and nerfed Mnemonic Potions with a cooldown timer to drink another, their output would be even more manageable)
Implementation Details
Because Spell Reserve abilities add little peak-power to the class, they're not worth spending feats. Instead, they are presented below as enhancements. Each is an active enhancement that casts a spell-like effect when triggered. For it to work, you must be able to cast a certain kind of spell, which means both having it on your currently-prepared list, and having enough spellpoints for a spell of that level. The power of the Spell Reserve ability is keyed off of that basis spell; DC is computed as a spell of that level, and damage dice are also scaled by spell level. Spell Reserve abilities have a school (used to apply Spell Focus DC benefits) and an energy type which applies to bonuses such as spell-damage amplification and spell crits. Some abilities make an attack roll: the AB is your caster level + casting ability mod + basis spell level + school focus bonus.
No metamagic can apply to Spell Reserve abilities. They are treated as 0 level for the purpose of concentration checks, and don't suffer ASF. The casting time is the usual (like a wizard's Cone of Cold), the cooldown is 2 seconds, and neither is altered by your casting class. Note that these enhancements are intentionally more-restricted to class than the D&D feats (if they were open to more DDO classes, they might need a higher cost for them).
Important option: It would be fine to add a cost of 1 sp per use of Spell Reserve ability. That would have little impact on gameplay, but might placate users who dislike the idea of a caster having "infinite" spells.
Fiery Burst (evocation)
1 AP, req wiz3 or sor4
If you have a fire spell of level 2 or higher available, you can detonate a small fireball on an enemy, inflicting 1d6 fire damage per spell level to nearby opponents (Reflex half).
Winter's Blast (evocation)
1 AP, req wiz3 or sor4
If you have a cold spell of level 2 or higher available, you can release a small cone of cold, inflicting 1d4 cold damage per spell level to opponents (Reflex half).
(Yes, there are no cold spells at level 2 currently)
Storm Bolt (evocation)
1 AP, req wiz5 or sor6
If you have a lightning spell of level 3 or higher available, you can cast a line of electricity, inflicting 1d6 lightning damage per spell level to opponents struck (Reflex half).
Acidic Splatter (conjuration)
1 AP, req wiz3 or sor4
If you have an acid spell of level 2 or higher available, you can hurl a glob of acid at an enemy. The attack does 1d6 acid damage per spell level, if an attack roll is successful.
Invisible Needle (evocation)
1 AP, req wiz5 or sor6
If you have a force spell of level 3 or higher available, you can launch a spike of energy at an enemy. The attack does 1d4 force damage per spell level, if an attack roll is successful.
Touch of Healing (conjuration)
1 AP, req cle2, dru3, brd4, or pal8.
If you have a Conjuration(healing) spell of level 2 or higher available, you can cure an ally for 3 hp per spell level. The effect is very short range, and only works on characters at under 50% hp.
Ray of Morbidity (necromancy)
1 AP, req wiz9 or sor10.
If you have a necromancy spell of level 5 or higher available, you can strike a target with a ray of deadly energy, inflicting 1d2 strength and constitution damage. On a successful fort save, she instead takes a 1d2 penalty to str and con for 30 sec (which doesn't stack with Ray of Enfeeblement).
(Note that Ray of Morbidity should be reconsidered if wounding or puncturing is nerfed)
Mystic Backlash (abjuration)
1 AP, req wiz9 or sor10
If you have an abjuration spell of level 5 or higher available, you can disrupt a creature's magical power into damaging energy. A single target takes 1d10 damage per spell level (Will negates). Creatures unable to cast spells are immune to this effect.