I already mentioned mission scripting, but as noted in Volition's replies, I want the ability to dynamically pull up and create some stuff on the fly. For instance, they mentioned no Fuzz in SR4 because it's a coding nightmare. Yep, I understand that fully, I can imagine it already. But if I wanted to make something vaguely similar, it'd need to be able to randomly create markers without having to be reliant on pre-defined points (or if it is, working from an internal list), and can randomly pick scenarios then generate the entities on the fly and any vehicles, as well as loading up the AI and any driving paths. I don't think I'd be able to have a mission where (thinking of a mission from San Andreas as an example) a guy you're chasing runs, steals a parked car to drive off in, hops out and gets into a boat, sails that about, then hops out and grabs a helicopter to take off in, but that'd be a great goal to work for. We'd thus need a way to have a way to declare a lot of this stuff in the lua scripts for this mission including what he looks like (I'm no modeller, so it'd probably be a pre-existing ped), the vehicles that are needed for the sequence, any AI shifts to make sure he can go from fleeing the player and going to a waypoint then shifting to jack a car and drive it properly to a given path, then hopping out and grabbing a boat and sailing it, and so on... plus of course spawning any specific vehicles in given variants and making sure the normal game AI peds don't suddenly jack them and mess stuff up...
I guess what I'm hoping for is the ability to declare a lot of stuff in a LUA on runtime without having to precompile it into the game map or game internals.