ToME WikiKhymChanur: Currently, there's only one mage staff artifact (the Mage Staff of Eternity), which is very rare, has a very high depth, and is very powerful. There should be at least a few mage staff artifacts that are more powerful than Mage Staves of Wizardry, yet less powerful than the Mage Staff of Eternity.
One way to do this would be to allow for randart mage staves. These would be like normal Mages Staves of Wizardry, yet stat boosts, resistences, immunities, and being indestructable. Intead of random weapon brands (since mage staves fill the weapons slot), there could be random boosts to various magic related skills.
For non-randart mage staff artifacts, some suggestions:
The Mage Staff of Storage (I'm no good at names): Like a combination of a Mage Staff of Wizardry and a super Mage Staff of Spells; allows you to store a bunch of spells, to free up inventory slots.
The Mage Staff of Geomancy: Gives a skill boost to each of the four geomancy skills. Activates for elemental ball.
The Mage Staff of Runes: basically a Mage Staff of Wizardy with a few runes and runestones embeded into it's surface. Runespells could be carved into the staff like on normal runestones, but without taking up inventory slots, and the embeded runes (from among the more popular/useful ones, like Life or Self) would also free up inventory slots taken up by runes. Could also give a boost to the Runelore skill.
Sentient Mage Staff: There should be at least one sentient mage staff (and probably only one). It would start out only as powerful as a normal Mage Staff of Wizardry, or perhaps even less powerful (since it will become pretty powerful with use). With each level gained, there would be an increment to the spell power boost, mana capacity boost, or number of spells storable. Additionally, with each level it gives a skill boost to a random magic school. This could be done in one of two ways: 1) each school is boosted by 3 to 5 points, and that's it; 2) each school starts out being boosted by only 1 point, but for every level gain for which the boost already exists, it gets another point. So, for example, after getting to level 5, the staff randomly gains a 1 point boost for the Fire school. But by the time the staff gets to level 10, the Fire boost is at 6 points.
Artifact Set Mage Staff: The above sentient mage staff could be a member of this set. The other items in the set could be gloves and a robe.
RogerBarnett: I agree. What use is a brand of Slay Dragon on a Mage Staff if you're never going to actually hit anything with it?
Solorin: Sentient mage staves have been discussed a little bit elsewhere - I am just reiterating a point someone else made in that discussion. But, how is the sentient mage staff going to gain levels if you don't hit anything with it? As a mage are you really going to melee with your mage staff?
With appropriately allocated skill points, how do mages do in the damage department? It seems to me that even with the appropriate skill points melee and archer types really need artifacts to enable them to deal the amount of damage required to survive at the higher levels. Do mages really need this extra boost?
Thematically, there really aren't that many mage stave artifacts in the books (mabye a stave for each one of the Istari, and a couple for Sauron and ringwraiths). However, if someone has some evidence to the contrary, I would love to be proven wrong on this point. Perhaps, randart mage staves could be powered up and made less rare.
kfoelsch: I was playing 2.3 with Tome Plus last night, and I found a randart mage staff... unfortunately, it wasn't any better than my existing mage staff of Wizardry... 
BurbLulls: I figured a sentinent mage staff could gain XP by casting the spell contained in it. With levels it would gain the basic resistances that sentinent realms bring, and instead of to-hit and to-dam rising spell power or mana could instead, but *slowly* (+1 or +%10 every 7-9 levels?).
SimonSorc: why can't a sentient staff (or armor, or whatever) share the XP of the player, alchemy-style? All the player has to do is gain XP for himself and some goes to the artifact instead. Seems simple enough. And it gives a drawback to overpowered sentient items (less XP for the player), too!
Atarlost: I'd kinda like to see a mage staff for the fighting mage or eru worshipping haftedmaster with boosted dice and a nice mix of low pval boosts to combat and spell power, maybe 2d4 damage and +2 to spell power and blows.
Logrusmage: i agree that more trueart magestaves are needed, especially a sentient one. as for gaining xp, my magestaff wielders usually do hit things with them, anything that's not worth bothering with spells for, so they'd gain xp that way, plus they'd gain it if you hit things with it specifically to level it.