I liked how i could summon stuff to run interference in IE games, some zombies or whatnot to hold part of the enemy camp busy while i clean up the other corner and get ready for the rest of the camp.
I did not particularly enjoy that it was all based on memorized spells, there was no cost for losing a summon, nor the fact that it became pretty annoying due to the long casting animation when you NEEDED to summon 4+ things to win an encounter.
Would love to see it tie more closely to the character itself, and from what little i have heard about the whole magic being connected to the "soul" of the character, it would make for a perfect starting point for a risk/reward based system. Granted, these are just my own personal thoughts and opinions, take it for what it is.
Base it off your skill, modified by passive abilities. Make each summon linked to the summoners health somehow, maybe detract X points of CON per summon(could be different depending on the summon strength, passive abilities could modify this number?), and only let you get down to 1, your HP will scale down according to your CON. You would end up being a pretty stout mage if you went summoner, but then your casting stat would be left down a bit... maybe this would be the perfect type of Mage to don some plate armor and carry a gun for backup?
Also, to make the summons more important to manage, don't give the reserved CON back if the summon dies. Forcing you to monitor and unsummon them before they die unless you want to lose those points until your next Rest. This would make the whole thing feel more important, and not so throw away. What would be even better would be if you actually summoned a specific creature every time, and that it would gain in strength with your summoning power. No more nameless skeleton nr1, instead you find a grave of some specific person who was, lets say, not nice in his lifetime and seemingly won't rest easy. Summon "Thorleif the Grim", your trusty, if silent, skeleton. I could even see how there could possibly be unique story based summons, with questlines attached.
Don't know, never was that into mages/sorcerors, since it was always such a hassle and extreme amount of microing... and very little actually "magical" stuff happening. More of a Monk guy myself, but i have always wanted to like mages, and hoped that some game would come along and make it really interesting to play a mage. I think Morrowind was the closest, thanks to its spellmaker and enchantment system, it really allowed to to go wild and make your mage feel really magical.
Anyway, some ideas on the matter anyway, have a good one!