If they were going to do it (and at this point I don’t think they ever will, sigh..) they should sprinkle them into other titles and see how they fare. Have Mantra or someone show up in Spider-Man where she/he is after someone and Peter has to stop her/him from killing someone, with Oeter not knowing Mantra is an immortal man reincarnated as a woman… see how people react. The problem with Marvel is that they’d try to turn it into an event with six one shots and a dozen tie ins with bookend issues. Just plant the seeds and let the characters grow and see if people like it. Treat them as brand new characters… If someone is popular enough to get their own book or limited series, play it by ear.
Again, it’ll never happen…