This was a popular topic in these forums back when Obsidian asked "If we (theoretically) did a Kickstarter, what would you like to see"? There were some great ideas there, but many of them were prohibitively expensive or impossible to implement.
This is just idle speculating for fun, but if possible I would like to see something new - i.e. not MMO/Dragon Age style cooldowns or D&D memorized slots. My own ideal (probably totally over-ambitious) magic system should affect the lore, and exist/be useful outside of combat. Something that bothered me with D&D - if you could manipulate wind, why could you only do one specific thing like "create a gust of wind"? If you could do that, you should also be able to
caress someone with a light wind at low levels, blow away toxic clouds, extinguish candles
create a strong gust that pushes a hostile away (or sweeps the street if spring cleaning) / do a smaller focused air punch to knock people out or break small objects at mid levels
keep a sailing ship moving / thicken air to slow down projectiles, hold someone still, or slow down a fall / fling heavy items at high levels
Have such power and control that you can fly / create an impassable field / sharpen the air to a killing edge / strangle someone at a distance / blast down walls / pick locks with ease at epic levels.
Magicians at epic levels would be feared or revered, a leader and a target for assassinations. When characters level up, they can choose to deepen an understanding of a domain (air, life, demonology, mind...), invest more points into power or finesse for a known domain, or put it in stats to get more stamina.
Casting spells drain stamina (or mana), so all spellcasting would be like sorcerers in D&D. You can risk casting higher levels spells than you are normally capable of or you have stamina left for - force-casting. This carries a high risk of knocking the spellcaster out, and/or physically damaging them. High level and really strong willed characters can choose to force-cast something that is likely to kill them, which can be useful both in combat as a dramatic cut-scene event.