I would suggest GURPS, the Generic Universal Role-Playing System by Steve Jackson Games.
Like Aftermath, it is classless, uses attribute scores and skill levels and point-based character generation. It also has a detailed tactical combat system, which if you use a map and read all of the advanced and the more realistic optional rules, is quite sophisticated and detailed. (Want to try to grab your opponent's weapon and wrestle it away from them, and then choke them? You want to swing at your opponent's weapon? Ok, are you trying to knock it away, or chop through it? There are specific detailed rules for that, and most other things.) There are also source-books which can expand the detail even further for certain types of action you might want to detail.
Also like Aftermath, some players find it to have nearly-unplayable amounts of detail, but in my experience, experienced detail-oriented players (or just a great GM) can make it quite manageable and enjoyable for players who enjoy detailed tactical combat.
There are space settings (GURPS Space) and actually Traveler-specific books too. I would say that Fantasy/medieval settings are one of the things GURPS does best, and there are many source books for different flavors of that.
Postscript details:
Since there have been some negative votes to this suggestion, and a request for more detail, I will add some points:
I have played a decent amount of Aftermath but it was over twenty years ago. I did review the Aftermath rules briefly for this answer, and although I don't have the energy or inclination to refresh and write up a side-by-side comparison as Dr. Brian Ballsun-Stanton suggested, I would say the combat systems are actually remarkably similar, and at very similar levels of detail and realism. My overall experience was, and my impression upon review is, that in many ways, GURPS is effectively as realistic or more so. GURPS is a far more elegant rule system, requires far fewer tables, and delivers much more playable realism per amount of work invested. The detail and meaningful appropriate cause-and-effect in a well-run GURPS combat using all the rules, is quite impressive. Aftermath does contain more detail on some points, but as an extremely detail-oriented player, I did not feel the need to import material from Aftermath, although it would be quite easy to use any details found only in Aftermath as spice for GURPS. I prefer the relative elegance of GURPS' mechanics, as well as the results in almost every way (except interesting details on some points), and feel it at least deserves serious consideration as pretty much exactly what the OP is asking for.
Aftermath does give an impression of being more detailed and serious by looking at the rules, because it is full of tables and charts, even a two-page flowchart of the procedure/sequence of play, etc., but someone who has mastered GURPS advanced combat will know that practically everything the Aftermath combat system covers, is also covered by GURPS in a comparable way, in some ways in even more detail, and with far less need for tables (or flowcharts), particularly during play. The style of the rules is notably different: GURPS uses natural language and is more accessible to casual gamers and gives room for different play styles and GM discretion, while Aftermath is mostly hard-core direct rules and tables like a wargame.
GURPS seems to often be played in a casual rules-lite fashion, in which case it can be very non-simulationist. Non-detail-oriented players may ignore and even not notice the detailed rules. Also, many of the worldbooks are mostly background overviews and have very little game mechanics meat in them. However, players interested in tactical detail who read and use the advanced rules and detailed optional rules, will find a very well-designed and detailed combat system, for which even more detail is available from the supplements I mentioned. For example, I'd say GURPS High Tech and GURPS Tactical Shooting can be used to get gun combat at least as detailed and realistic as Aftermath or even Phoenix Command, yet more elegant and easier to play.
