I don't see a reason that it couldn't be under the same heading. Games like Fallout are rather defined in their setting. In spirit with how it was presented, Worlds of Ultima was a good example of taking something to a different setting but using the same tech (Ultima 6).
Games like Fallout also tended to have a tightly interwoven storyline. JA doesn't seem so, as much. There's what happened to the old mercs, here's some new, and this is actually a good thing for a game to possess same name, different setting, distinctive sub-name, but with the same good quality. Flavor is good, but only with meat. The choices of mercs and morecould easily reflect the setting, and I believe the humor used in the Sir-Tech games should continue, Wiz 8, JA2, etc.. You could also choose to play in Sci-Fi mode or not in JA2, which basically opens up two distinct storylines in the same game. That suggests that the exact details aren't important (as long as they are congruent in each time they are presented), but as long as the details are good and fun to play through.
As long as it turns out good and polished, there shouldn't be any reason to worry about what it is called. If it's good, then it should be worthy to have the name Jagged Alliance on it. It's a title/base concept/engine spin-off, rather than a setting spin-off, so it shouldn't have much adverse affect upon the core franchise, much like WoU. If it has the same good construction and overall style, then it wouldn't be seen that unfavorably unless someone didn't like the setting, although game mechanics, construction, and general composition of the game should be enough to overcome that as the person did expect a game worthy of the JA franchise.
OS is a totally different kind of game in (supposedly) the same setting. JA:Whatever will be the same kind of game in a different setting.
