As for an RPG, I'd say one of the main issues would be the story you want to tell, if you emotionally relate to the characters and so on. You could make a game based on the original XCom game where you're in charge of a single squad and its members through a series of missions, where the player decisions are more related to which characters to recruit, their relationships to one another and how to develop their abilities.
This is a Bioware RPG you're describing. The genre is far wider than that, man (and I'm sure that what Yankes meant, too). The story can be relayed in various ways, not only by dialogue.
The series of pre-scripted missions isn't any better at telling the story than an open world, if said world has enough story and background going.
The inter-character sheaningans aren't neccessary at all. At all. They might be a part of the story, they might not. In the old RPGs the characters were a band of comrades, your faithful soldiers and companions... and also, they often could die in the random vagaries of battle, without immediately bringing up the Game Over screen - like in XCom.
The high-adventure genre is certainly most well known (to the point I'm fed up with it) - all the more needed are under-represented genres, such as sci-fi adventure, cyberpunk, or even less represented - military, alternate reality, hard sf, science fantasy, detective.
Advancement decisions are indeed taken, but on the equipment not skills per se - sure, everyone can become a supersoldier in OXcom, but you still have something to say.
One of the things OXcom is lacking is a finite but complex storyline where a string of missions, and unique characters/items can appear. But this WILL appear since TFTD forces it
Another is more character customization... sadly this is impossible in the boundaries of this project, but absolutely not unheard of as far as similar games go (Jagged Alliance, UFO Afterlight, hell, even UFO:ET, Xenonauts and XCOM:Apocalypse were trying to make the characters more personal.
Also... "Roleplaying" XCom isn't anything new, like these people of old who hacked game files to get a 2-seat capacity for interceptors - for no other purpose than to roleplay pilots