While Fall From Heaven game *could* use the Civ V engine, I think it is more likely that they would borrow the engine Stardock will use for the Elemental game. Although, of course, making their own engine is possible, im sure, if certainly a lot more work.
Fall From Heaven was only limited by the game of Civ IV (and possibly that engine as well). While there have been modmods that make FFH seem more like a Civ counterpart (Orbis), the base FFH is, at its core, completely different from the game Civ IV.
There are two directions (in my mind) that they could take. One would be a Dungeon Crawler/Adventure game, and the other would be an Empire Builder.
Again, if its an empire builder, I hope some notes are taken from either King Arthur or Elemental, in terms of having a turn based strategy blended with interesting battles. Multi-path quests, that follow a story line (on the battle map!!) also sound interesting.
Personally this RPG-esque Empire builder with an integrated Quest system sounds like the best option. Especially if it could somehow be made multiplayer compatable.
If, however, it was a Dungeon Crawler ... then I would at least hope for a creative magic system, as well as game-play somewhat reminiscent of recent elder scrolls titles (moreso Morrowind than Oblivion).
Also, if it was to be a Dungeon Crawler, I would hope for more than isolated 4-man quests like you see in most RPGs, while wars are going on. I would, instead, at key moments, expect you to control actual armies against the opposing forces. (Sure, you can only give "specific" commands to your-self or your party ... but, like in Joan of Arc, you can command the rest of the troops about the battle-field via movement commands and formation commands).
Basically, either way I would like to see this an integration of Empire Building and Questing, potentially following a storyline, although open-ended is marvelous as well.
It doesn't need to have the graphics of "contemporary games" ... at best it can be like over-sized mod models, which is also roughly equivalent to the Morrowind game. Although I wouldn't mind if its graphics were of the level of Final Fantasy VII.
The main difference would be wether the interaction with the empire-building was from a 1st person, human-human interaction, or on a 3rd person view where you are constantly in the world map, and actually building your armies and cities.
For the empire builder, you would actually be controlling your religion, politics, population growth, city building, army training ... actually controlling your empire on the macro scale.
For the "Dungeon Crawler" you would start out as a commoner (or prince), in a pre-existing city ... due a few small-time quests for fame, and begin to accrue political power and prestige on the personal level, acting more as a Champion of the people (general of the empire's armies) than the Emperor himself. In this way, I would see something similar to the Suidoken series, only scaled up to levels of Awesome, with Total War-like battles.
Actually, in both versions I would like for total-war like battles. The main difference is that in the empire-builder the armies would have the *potential* to get bigger ... because each unit has its own exp, while in the Dungeon Crawler each soldier has its own stats.
Actually, even that problem could be solved in both forms. Regular soldiers (in the Dungeon Crawling version) would be reserved to their units (unit shares Exp, individual soldier has his own HP each) ... and they would wait in the "Main Camp" which is only saved for pitched high-level battles against the enemy. Meanwhile your Heroes/Champions ... those in your Party (and perhaps some generals in the main camp) would have their own stats n HP, on the individual level, and would almost always be stronger than a regular soldier.
In the empire building version of this, soldiers you train share Exp (tho individual soldiers have their own HP, even within a unit) ... and there are special people "Champions" or "Heroes" that can fight on the battlefield against many weaker opponents ... with more complex stats, and can wield special items you may find in your "questing". In this case, people of the royal family could potentially ride out to battle as well ... including yourself (which would be amazingly awesome). In King Arthur, Knights of the Round functioned as these Heroes (also as Generals) while the King himself, unfortunately, was not able to partake. However in Elemental the King is a powerful Wizard that can (potentially) defeat an opponent's army single-handedly. Either way, these Champions and Royal figures can double both as Questing Adventurers, as well as Generals for your army.
So you see, a lot of the concept and mechanics is some-what the same ... however the perception of events, omnipresence vs singular presence, and other basic *external* elements of gameplay are different.
For instance, although the questing and battles would be some what similar in both incarnations ... The constant strategy of maintaining an Empire can not be matched in an RPG adventure game, and the depth of the Dungeons and Puzzles of the questing cannot be matched in an Empire Building game.
Essentially, I would prefer either a FFH (empire) with proper battles, and some sense of Questing or Story-line, or some sort of Morrowind with proper battles and some sense of political power and responsibility (as you gain the respect of the government(s)).
Another difference between Empire Building and RPG adventure game (since the battles and armies are "mostly" the same) would be technology. RPG adventure would take place in a relatively unchanging realm, with real changes only occuring with the recruiting of armies, the "potential" destruction of cities, and the changing political control as one faction takes over the other's fort or city.
Meanwhile, the Empire Building version would take place over a rather longer period of time, be slightly more abstracted, and contain an organic growth of empire (as well as technology). You might start out with clubs, and then eventually have powerful magics and powerful weapons ... while in the RPG you have a little bit of everything in a more permanent and (seemingly) realistic world over a relatively short amount of time (several years) .... while in Empire mode you would be playing over potentially hundreds of years.