First off, I've never read the Sailor Moon manga, but I did watch the anime when I was younger. I know they're both different in a lot of aspects, but it's hard for me to get back into it since my obsession ended so long ago.

SO! I'd like to find a manga similar, but a little more mature. I really like the superhero(ine)-feel Sailor Moon has.

More than one main superhero(ine) is a plus, too. 🙂

Cheers.

oh, dear, THE question.
problem is, Sailor Moon was the first in its genre, and, as flawed as it was, everything else that came after always feels like a copy. There's Tokyo Mew Mew, but it's not more mature than Sailor Moon... and frankly the manga version of TMM is poorly done.
Maybe you could try Cutie Honey, but I don't know if there are scans available of it.
In the end, I'd suggest you to try the Sailor Moon manga. It IS more mature than the anime counterpart ever was. Endure the lack of consistency of the art and some 'rinse and repeat' of some plot devices: what is left of it is still good and enjoyable even now ( I read and enjoyed it a lot when I was 14, even now I still do -differently from the anime- . And I'm as twice as old ;p ) .

Motomi Kyousuke is DA (WO)MAN ;D
http://www.mangaupdates.com/showtopic.php?tid=14467&post=382161

If you liked SM you'll probably like Card Captor Sakura, another magic girl series but more individual than a team one. Also it's well written and beautifully drawn (CLAMP fangirl), I would put it a bit more mature than SM but not hugely.