Something that is very open-ended. Sort of like TES4. However, one thing I've been thinking of lately, with regards to Neverwinter Nights 2, is a game with two parallels of the same story. What I mean is to have the typical "good-vs-evil" story, but depending on your disposition you can choose which side to fight for. Essentially you'll have 2 different endings: one where the good vanquishes evil, and the other where evil achieves it's goal of world-domination or complete destruction. What does that have to do with NWN2 you ask? Well, it seems that no matter what your alignment is with NWN2, you ALWAYS have to run the course of saving the world. A bad alignment just makes the game harder because nobody's willing to help you. A variation of that idea could simply be two sides of a war (sort of like the 4 races of EVE-Online) where everybody is neutral with everybody at the beginning, but depending on their actions can end up fighting for one cause or the other. Simply, it's an example that the only thing that separates enemies is their perception of what is right, not necessarily good-vs-evil.

Other than that, a solid (back)story and some romance if you can fit it in smoothly wouldn't hurt it.