I'm sure you would be welcome to upload here at Leefish if you wish. We may not have a huge audience or be giant Sci-Fi fans, but we're a friendly bunch... and we do have at least one big Star Trek fan here, whom you probably already know.
And actually, MTS has undergone some big changes with regards to uploading lately - they're trying to be quicker, more accepting and more welcoming to folks who want to upload there - that might be a good match with you right now, since both your attitude and the site's attitude have improved. I'm sure they'd welcome you back as long as you play nice and perhaps introduce yourself the way you have here. They also have a social groups feature there. It's kind of like a private discussion forum that you can set up and invite people to join, so you could maybe set one up for yourself and your fellow SimTrek "refugees". You can choose to either make it "members only" or make it public so that anyone is free to read and join.
Tumblr is also a fun place for simming stuff. All you have to do is tag your things properly ("sims 2", "sims 2 download", "star trek", etc.) and people will find it. In fact there are lots of Star Trek fans on tumblr - it's a good place for fandom related stuff (not just Star Trek but lots of fandoms). Many folks complain that the Tumblr community is not friendly, but I've been there for 3 years now and have recieved only 1 or 2 rude messages in all that time. Be nice, follow nice people's blogs, and negativity will stay far away.
I agree with Fansee - good tagging, and some regular updating make Tumblr a great resource for uniting fans with each other. No matter what else you do, putting pictures up on Tumblr is a great way to connect.
If MTS is feeling a bit daunting, why not try a 'testing of the waters'. Look for a Star Trek forum on there (I suspect that there would likely be one existing), or make a star trek related post in there somewhere. Make a star-trek related upload, and watch who comments. If there are no comments, then there's a clear indication that setting up a forum there won't be worth it.
In the meantime, hang out here! Nix isn't the only Star Trek fan (although my Trekkiness hasn't really ended up in simming except as a wish to do a trek hood sometime, like many of the other themed hoods I want to do).
There's really two issues - one of social, one of creating. I actually find Leefish to be great for both.
Leefish can be a place for you too - again, I try and overcome my tendencies to being a control freak; I don't always succeed, but I try.
Tumblr; I dunno, I sometimes think that Tumblr is going to have to change in order to grow, there are a lot of aspects of the tumblr set up I don't like and some parts of the community there can be a bit too competitive for me. I have a real thing about popularity contest type behaviour.
Ofc, Leefish is also an awesome site, and you'd do very well to settle in here too.