To answer seriously, basically Mima is cool for the same reasons Marisa is, except she replaces petty theft with outright villainy. She's basically a stereotypical cackling witch, which is pretty hilarious. She also stands out by being one of the more mature characters in terms of (apparent) age.
I also like Mima for having an interesting relationship with Reimu. As the PC-98 series went on they were increasingly close with each other, although they still technically had reasons to be enemies. It's a bit more involved than how most Windows antagonists simply became acquaintances once defeated.
I like to imagine that once Reimu introduced the spellcard rules, Mima was like "fuck this casual shit, call me when we're playing for real" and left. :V
Although, I'm not sure I'd be as excited about her return as other Mima fans would. While it'd be interesting to see what Mima has been up to all this time, I worry that if she came back in any active capacity Marisa would have to go back to living in her shadow. Some people, myself included, feel that Marisa was meant to become the new Mima, although she's still a long way from that, I think.
Mima off doing things by herself would be great though, I'd love to see her start shit with Yukari or Byakuren or something (despite probably being woefully underpowered compared to them by now), or team up with Yuka again to go trolling everyone.
