This looks good at first glance, but let's take a look at how this actually works out in the game.
Let's say we have a first life wizards who is focused on CCing enemies with dancing balls and mass hold monster. This means that every spell he is using has will save. So he might be able to hit enemies with a low will save (even this takes quite a ot of investment), but everything else is a major problem. And don't forget that there are all the undead and a lot of other enemies who are completly immune to these kinds of spells.
So instead of being able to do at least something in every quest, this wizard doesn't have a whole lot to do the majority of the time and he is relying on his party member to actually kill stuff.
Also because having spells hit 95% of the time is so much better than let's say 80% (it means that they fail 75% less) most players don't actually focus that much on different spells. They rather max out 1 spellschool and usually it's necromancy because instant killing is pretty powerful, the number of useful spells in this school is pretty big and the spells work in a variety of situations.
The best enhancement for spellcasters imo is the storm of vengeance SLA for the druid. Not in terms of how much power it gives the spellcaster, but in terms of design. It has a very long cooldown so it can't be spammed like a lot of other SLAs, but it saves a lot of sp. The long cooldown doesn't discourage as much from using it as some other abilities with long cooldowns do, so it actually feels good to use it. You don't have to save it for the next room, because even if you need it there you can still use the actual spell.
This mainly works for damage spells of course because they eat a lot of sp through metamagics and they are used in almst every situation. Utility spells like invisibility don't work well with SLAs. They are not used often enough.
What might help CC casting would be long CD SLAs with increased DCs.
This kind of powerful high level spell is missing for most spellcasters. Instead of getting really powerful spells where the caster has to wait for a good opportunity it's more about spamming out more efficient low level spells.
So yes casters should be very powerful, but they should have to manage their resources.
Something else people have asked a lot for in the past are epic spells. It would propably work out good enoguh if there were epic versions of already existing spells that don't scale well enougn in the epic levels. Spells like wall of fire, stone skin and rage come to mind.