Yeah, you could do that. It's mainly that you're dividing it into parts, as you put it, that would make it work. That is sort of similar to what they did in Shadows of the Empire. They used the different types of engines for the different mission types, FPS engine for the shoot em ups, & the space sim engine for the ship flying stuff. In a SP game with a linear plot this would be very easy.

But a SP or MP game that allows the player the freedom to choose FPS or Flight mode at any moment during the game without loading a new mission would require a different type of engine to handle both the huge maps necessary for the flight mode aspect along with the incredible amount of detail required to inhabit that entire map with the kind of detailed buildings, people, vehicles, etc. that you find in a FPS. It's years away from reality, but when it comes it's gonna be way cool.

Not years away from reality!
Just look at BF1942 man. IMO that game has very nice graphics, runs fairly smooth, and has pretty nice multiplayer (multiplayer is a little laggy for my comp, but I have a crap GFX card).
The splicing wouldn't be splicing at all. It would just be having two different games rolled into one. If you did something like this it would basicly be like having JA:Space and JA:Ground. You join a server playing a space map, it loads with the JA:Space engine, when you join a game playing a ground map it loads the JA:Ground engine. There are two problems with this:
#1: You are practically doubling the game size
#2: From a marketting stand point, why? You can sell them as two games. They would have more people buy both than they would buy one, even though some people would be attracted by the game having both.
I hate to say why, but it probably just won't happen. It saddens me though, they will probably have snow speeders or something though. I think I said 5 axi here earlier, I meant to say 6.

P.S.

Actually, the map wouldn't have to be that large, and it could be done a different way. You just make the spaceship you are piloting stationery and allow it to pivot in any direction, and have a particle system to generate a moving star-field, which speeds up and slows down according to the speed of your 'ship'. Then you just introduce other ships as targets to shoot at, which could always come at you from an angle so you never physically see them spawn into the map.

Nah, this way you have to reprogram it for multiplayer, and if you have a stationary star destroyer you have to have it move according to what the player presses. You have to calculate other things moving according to the way you move, plus, according to the way they move. It's much easier to calculate where you're moving seperately.

Yeah, but we're talking about combining them so they're both on the same server as a single game type. If you're in a craft, you could do strafing/bombing runs against other players running around on the ground. If you're on the ground, you could shoot at the craft with small arms, turret guns, or a shoulder fired missle, etc. Players could board a craft & one of them fly it to another part of the map where they all get out and start a firefight. Or they could disable another ship & board it, eliminate the crew aboard & take over the controls. It would all be intermixed, with people in flight mode & FPS mode all at the same time playing with each other & seeing everything the others are doing, all without knowingly switching to any different game type.

This type of game is a few years away to get that kind of detail. We're talking maps the size of Texas with every single building of every single city or town being a fully detailed map of its own complete with AI civilians etc. We won't see this sort of thing until the average pc has the technology of today's supercomputers, & even then we may have to wait a bit. Will be darn cool though when it happens.

All they need is a terrain generator... then they could do it like tribes 2. Just have an "in-bounds" area surrounded by something to limit view distance a little. You wouldn't want to go out of bounds because nothing would be there but land, no people.
Look at tribes 2, granted it doesn't have all of the features of this game, but that was two years ago, and their graphics are still satisfactory today.

That's a cool idea. I've seen some flight sims that have similar maps, once you reach the edge there is no terrain, etc.

Tribes probably has an algorithm to generate terrain, and the algorithm is constant for each map. The terrain outside the bounds of the map can be generated on the fly from the algorithm, and it'll be the same for all clients.

That would work perfectly for this, just a few modifications to make the terrain look like star wars terrain...

Tribes 2 style would be probably the best way to do it. That would mean Infantry vs. Speeders, though. If you could expand it with a similar generator to the outside-bounds one to include space, you'd have everything you need. The dude runs to the spaceport, hops in his X-Wing, takes off. He can watch the couds part and he's in space, which would have a better framerate than the ground based part, I think, and you could have dogfights going on as the AT-ATs move on the rebel base. The only problem is that you wouldn't have capture-the-ship gameplay where you actually play a stormtrooper and you have to move through the ship. To remedy this, the ships' insides could be premade mini-maps that would load up as the docking operations commence. The players in TIE fighters wouldn't need to load the map, but the Stormtroopers would. I'm not sure how that could be acheived, but it seems quite possible. This is the kind of game I dream about; where all the great aspects of SW roll into one great big immersion.