Hello all,
What I'd like to see a magic-using character do (either innately or through a prestige class):
1) storing spells - and I don't mean crafting. I'd like such a character to be able to imbue his equipment with spells, so that they could be cast with a minimum of drawbacks like attacks of opportunity. One piece of equipment for each major location (one weapon, one amulet, and one ring, for instance, to a max of class level/2) could "hold" a spell for a long time (one hour per level, for instance, and the spell is lost if not used during this time frame). The spell is effectively cast, removing its slot from the regularly-cast spell daily allotment. Advanced version (gained at level 8, for instance) could allow to store more than one spell in one item, and activating the item (a standard action like tapping a staff on the ground, for instance) releases all spells at the same time and using the item as point of origin (but in a given order, if, say, you had Protection from Fire and Fireball stored at the same time).
2) runes. The character would be able to scribe words of power, either to imbue an item with magical power (see 1), create a trap, etc. There would be a given set of runes, each with a significance, and associating them would create magical effects. Some runes would require the character to have a given number of class levels. The complexity of the rules formulae would also depend on the class level (examples: one-rune formulae at level 2, and one additional rune every two levels). There would be a listing of most rune formulae, but not all, thus encouraging creativity and discussion with GMs. The number of runes and/or formulae used per day would be limited and a function of the class level.
3) additional options for polymorph spells/shapes. Vermins, aberrations, etc.
4) a "nature" option for those "greenish" magicians (as well as some druids). This would allow them to unleash spells as wild energy (or "force of nature"). Rules-wise, this could either look like Channel Energy (targetting anything non-natural), or something else. Upon a failed save, non-natural items could be reduced into their natural components (a metal armor would transform into chunks of metal ore and fall on the ground). This could also improve nearby natural life (animals and plants, and this could also improve their attitude towards the character).
5) a "technology" option for those "civilized" folks. This could be a couple powers to enhance or disable constructs and other mechanical items (like dungeon traps). Could also include a Channel-like effect to do so in a given radius. Could include a bonus to Crafting checks, especially when creating various types of constructs.
6) a couple spells to make the magic-user more proficient in/against specific melee tactics - not all at the same time! For instance, a Verbal-only spell to Bull Rush all adjacent creatures 5 feet (or the ability to Channel wild spell energy into such a Pushing -or Dragging, or Repositioning, etc- effect).
7) I won't like a book about new and powerful magiks without it containing ways to reestablish balance. So, let's include new divine spells and/or abilities to use or resist the new and improved arcane abilities (runes, etc). Same for non-casters: include tactics/feats/items to allow them to resist the magical onslaught somewhat. In that regard, I can easily imagine a feat giving a once-per-day +10 circumstance bonus to one roll (whether it's Initiative or Saves).
I'm quite sure that these ideas have been raised before, but I wanted to group them here.
$0.02