I wish they had more diversity in the Destro Staves - Elemental is nice and all but some times it just doesn't fit the theme of a character, I wish they would add:

Shadow Staff - They could reuse a lot of the animation they have in the Nightblade powers, so it wouldn't be difficult to do.
"Light" Staff - a staff whose effects look like the Templar powers Gold Light, again easy to do and could share some of the animations.
"Negative Energy" Staff - it is already in the game, its the ones some Daedra and Necromancers use that look like blue translucent energy - several of the Warden Powers use a Similar effect.

With the exception of the Ice Staff a Magic Tank is very limited in weapons it can use that really benefit it and work thematically, so why don't they Add:

Wands - single handed versions of the Staves that can be used with a shield. Probably would do less damage then its 2h counterpart but It wouldn't be that hard to change some powers and make one a taunt (if you just wanted the wand as DPS you could just skip over that specific power).

The big one a lot of people I am sure ask for is Hand to Hand, and I figured a way to make it viable for either stamina or magic characters...

"Brass Knuckles" for Stamina and "Claws" for Magic - You would buy one or the other weapon style, or heck have one bar of each if you wanted to. They would use the same animations and same weapon skills, only difference would be what the heavy attack recharged. Out of all the ones I listed this one seems like it is a no brainer and should have already been done at this point.

All the ones above aren't really that much of a stretch using the stuff in the game, this final one might be, but would still be cool.

The rediscover a crossbow designed by Dwemer that fires bolts of magic instead of arrows, this would allow Magic Based character to have some of the fun Bow characters do without suffering in the power recharge area. Might not be exactly the same as Bow skills, but they could get pretty close. In fact in the game the one Fighter Skill Line power "Silver Bolts" already makes use of this type of weapon, and the graphics it uses shooting out of the crossbow looks far more like bolts of energy then physical bolts, especially if you choose the Morph that fires multiple bolts at the same time. So the base animation is in the game, they would just have to expand on it, but it would be a really cool Magic Weapon.