Depends on what you mean by "better". If you mean more successful, then I guess fancy it up and pretend you have a plot. If you mean better to me, I am just not that into the idea of participating in a madoka magika fandom at the moment, though as I said, if roleplaying magical girls is enough, I may have an idea.
If you mean better in general, I'd suggest sticking to the setting but changing the story. Try to keep the essential of madoka magika, but don't try to copy it's plot and curves or you will fall flat on your face
Well, you can't magically make people join what they are not interested in, and interest comes in waves. Either you're lucky enough to catch the right wave or you weren't dealt the right hand. Personally, I think trying to make things in a way that attracts people is a waste of time. Try to do them in a way you yourself find right instead. That's what I do anyway.
Alright. In the end what you do, that's your decision. I can just share my method, and honestly, I think I stand as an example of what I say. I have a very unnusual way of going about things that aims at making them good as I see them, rather than popular. And now I have a VERY active RP going full of people who actually have shown an appreciation for how I do things.
Honestly, I don't think I would have been happy giving up what I think is right and half-assing it just to get more attention, if I had even managed that. This way, I can be happy that I got what I wanted, even if it took me dozens of tries.
maybe I should be more specific on what's missing from basic information? Like, what are the characters, what space/time relation does this have to the cannon, what tone you want, post requirements, other important rules and such