It depends on the stakes. While I think all three of my current characters are invested enough in the Turtle (and have nothing pressing at home) to be comfortable with hanging around*, as a player I want the problem to be as compelling.

Like... I'll use Zelgadis as an example. His initial involvement in plot was more as 'I don't trust Eshai or Evandu, but I am no stranger to cosmic evil, so let us investigate' dovetailing with Asti making a strong impression on him. Because of that, he's become a lot more invested in Keelai to the point of becoming a regular at the hospital and in his neighborhood. So he now has personal stakes against Malicant in a way that he didn't on application.

While I can retire characters whose arcs have completed and app new characters, ultimately I want some continuity and some interest in the plot. And some feel for similar stakes. Like, to use your example, Bane and the Riddler have different power sets and MOs but ultimately, both are serious threats to Batman and/or things Batman values (though maybe not the same things). If you went from Bane to a villain Batman could address by (as Bruce Wayne) making a call to Commissioner Gordon... well, either it's a short story or the audience wonders why he isn't. Or, say, a problem in Metropolis where we wonder why Batman isn't letting the locals handle it.

I know that's not helpful from a mod perspective, since it's not something a mod can hand me on a platter. I have to figure out why my characters would find this compelling. Interfering with the kedan in 'their' city is a lot less morally clear-cut as Malicant, which is a factor.)

(It also has to answer why Evandu is still having people brought in. 'We have little choice' is morally gray but compelling for a Malicant scale threat. More internal threats are more likely to cast Evandu as an antagonist. Which I wouldn't be opposed to, but needs to be addressed.)

* As opposed to 'well, we killed the thing you wanted us to kill, now back to our lives and our own problems'.