To be honest, I'd love to see FT incorporated into SCE. I don't know how much work it'd be, or what the licensing conditions are. From a user point of view, tho, it would be great to be able to switch to truck racing without exiting the game; it would also make it easier to race trucks on the myriad SCE addon tracks available online, and it would also save some hard disk space (as we wouldn't have to dedicate space to duplicate tracks). It's far from a #1 priority, but it could work perfectly as a DLC (paid DLC for SCE owners, heavily discounted or free for owners of both).
That said, FT is still young in the steam marketplace, and there's probably a few (a bit foolish, imo) users who may only have bought FT and not SCE, who would surely be upset if the FT game was left aside once it became a DLC addon for SCE, so Reiza would still have to support both games (GSCE+FT addon and FT standalone) separately, anyway, at least for a while.
Either way, I'll certainly keep playing and enjoying both (SCE primarily, FT for a change of pace) for a long time, still.