I would like the system to have nice incentives (or penalties for not) for PC to purchase stuff other than adventuring gear (a house, for example).
I'm brainstorming for this. Working title: "What is Best in Life?"
I wish there was a game with a more interesting system of magic. I'd like to see something where magic follows certain consistent rules, which in turns mean you could customize spells on the fly. I don't want to cast fireball by saying "I cast fireball," I want to combine spell elements to create a sphere of fire which is hurled at target X.
ETA: It's possible that there is a game that does this! My knowledge of RPGs is far from comprehensive.
I know we're not supposed to mention existing systems to start a fight, but you might be interested in this.