I'd like to preface this post with the fact that until reasonably recently, my ONLY serious toon was a capped drow air savant, and I LOVE playing casters.
Single target DoTs, as they are now, are severely overpowered. Think about it. As a caster, I can cast a spell that hits for the same damage as all my other spells 8 times, for exactly the same sp cost. They are the single highest DPS a caster can pump out while simultaneously being the most SP efficient spells available. Casters were always meant to be the highest DPS classes in the game. If they weren't, there would be no purpose in bringing along more than one per raid/group to serve as a buff/hold dispenser (sound familiar?). However, their DPS isn't meant to be sustainable. It's meant to be something that, if you go max dps on something, you run out fairly quickly. DoTs have made it so that we don't run out when in "Max deeps" mode, which is screwing up game balance because we can afford to out dps EVERYONE and we can keep doing it for minutes at a time.
The solution, in my honest/humble (which does the h stand for? meh.) opinion, is to decrease the damage of DoTs significantly while keeping the best available sp efficiency. I'm not a game developer, so I can't give exactly how, but I would reduce it so that a cast of a DoT does about 2-4 times as much as a normal spell does instead of 8, but extend the duration by about 1.5. The result would be an extremely SP efficient spell for contributing dps during long fights without blowing everything else completely out of the water.
Too prevent a return to the horrors that were pre-u9 caster social mechanics, I would then give the casters a ridiculously high damage, high sp single target and/or aoe spell(s) (talking something in the order of 2-3 times as powerful as the average polar ray, which in my experience hits for around 450). This would be something to cast when you don't care about sp efficiency, you just need to do as much dps as possible. This spell would be superb for dealing with trash or playing with agro mechanics, but would not enable the caster to maintain their DPS for long enough to lead to significant imbalancing at the long boss fights.
My final suggestion would be to balance for casters who dump their casting stat (mostly relevant to fvs). Make the difference in SP more noticable between classes and when a casting stat is dumped. Non-wis based fvs would already be losing the benefits of the new Überspell, since it would be save-based, however, the difference between dumping your casting stat and maximizing it should NOT be a few hundred sp. With my suggestions, sp conservation will be a lot more serious, and the player skill involved won't matter at all considering how pretty much everyone can manage similar endgame sp count. Instead of having each point of the stat cost 29 sp at level 20, make it closer to 60-70. Also, FvS and Sorcs should be getting a significant boost in sp over Clerics and Wizards. In the FvS/Cleric case, with the new importance of having high sp, having a split casting stat will become incredibly hard to manage, and so should be balanced with that in mind (especially since FvS are generally more inclined flavor/RP wise as offensive casters). For Wizards/Sorcerers, the new expensive spells will be DC based, which means that the Sorcerer (the class that should be the best nuker) will be getting a penalty to dps due to their lower DCs. Being able to cast the spell significantly more times should counteract this when compounded with a faster cast rate. I was thinking a good adjustment (again, not sure of the numbers exactly - I'm no dev) would be to change the average difference between wizards and sorceres from 700-800 (around 2100 to 2800) to closer to 1500 (around 2000 to 3500). Even with these changes, a rich enough wizard/cleric (plat ot RL money) will be able to outcast a fvs/sorc with those freaking annoying potions (HATE these. As someone who likes to collect things, I just have a stack of these gathering dust that I'm too scared to use in case I need them more later.)
TL;DR:
Nerf DoTs from being 8x normal spell damage to around 2-4x, and extend their duration to make them good sp efficient choices for long fights.
Give all types of caster new high level spells that do huge amounts of dps (2-3x your average polar ray) for higher sp costs, to reinforce a casters role of best non-sustainable dps. Should make fighting normal/epic trash with straight damage spells (which is STILL not done, usually wail/hold beatdown) more sp effective.
Increase the effect one's casting stat has on SP to help with balancing SP conservation mechanics, and increase the SP gap between the spontaneous casters and their counterparts to help balance out disadvantages (because dumping the sp stat will hurt more, clerics have an advantage over fvs; because the new top dps spells are save based, wizards have an advantage over sorcs; because fvs and sorcs should be doing more damage with these new spells, they get to cast them more often).
Sorry for the long post, and I hope the TL;DR covers everything (though shouldn't it be TL,DR? The grammar nerd inside me says that the semicolon doesn't belong).