Will there be an alternate universe after Secret Wars?

[Habis]

That's considerably better, sure... but if you change things in the present, do those visiting from the now erased future get erased as well (ala Back to the Future, if not out of all memory), or do they persist as temporal anomalies like Rachel Grey/Summers, evidence that their alternate futures, even if now erased, once existed in the sense that they could send travelers back?

Whatever the writer wants, I guess. No matter how you do it, you would be creating a paradox:

-If Rachel comes back and prevents her own birth but she survives as an anomaly, that's a paradox.
-If she gets erased but the effects of what she has done (including memories of her) remain, that's a paradox too.
-If if she gets erased and the effects of what she has done get erased too, then in theory, she couldn't have prevented her own birth, so she wouldn't get erased, so she would be back, so she did prevent her own birth, so she would get erased, so she couldn't have prevented her own birth....etc., repeat ad infinitum...

Since everything would be a paradox, just pick whatever one you like more...Cyclops vanished in Uncanny X-Men, but Wolverine remained in Age of Ultron (and there were two Wolverines).

And what does this mean for characters like Kang? Does he risk losing his power base in the future, if not his very existence, every time he comes back to try to conquer the present (whether that be the present of Earth or Battleworld)?

He becomes vastly less impressive. He would rule an interplanetary empire in the future, and would attack the past knowing that he would be erasing his own present (but in exchange, his empire would start earlier and grow bigger). I guess he must have some means to maintain his existance as a paradox even when he changes the timeline, because he isn't afraid of doing so.

[vitruvian]

Whatever the writer wants, I guess. No matter how you do it, you would be creating a paradox:

-If Rachel comes back and prevents her own birth but she survives as an anomaly, that's a paradox.
-If she gets erased but the effects of what she has done (including memories of her) remain, that's a paradox too.
-If if she gets erased and the effects of what she has done get erased too, then in theory, she couldn't have prevented her own birth, so she wouldn't get erased, so she would be back, so she did prevent her own birth, so she would get erased, so she couldn't have prevented her own birth....etc., repeat ad infinitum...

Only your third point (she erased herself, so she didn't come back, so she wasn't erased, so she came back, so she erased herself... ad infinitum) is actually a paradox per se... the previous two options are possible solutions to the paradox, namely the 'Erased timeline hypothesis' where a time traveler persists as an anomaly despite having wiped out their future timeline, and a weaker form of the Erased timeline hypothesis where memories but not the traveler themselves persist.

There are a bunch of others that have been used in SF stories and philosophy courses, many of which Marvel hasn't even played with yet.

He becomes vastly less impressive. He would rule an interplanetary empire in the future, and would attack the past knowing that he would be erasing his own present (but in exchange, his empire would start earlier and grow bigger). I guess he must have some means to maintain his existance as a paradox even when he changes the timeline, because he isn't afraid of doing so.

Or he knows (or believes) that the Erased timeline hypothesis is true... or maybe it used to be the branching timeline hypothesis, and he had a way of getting back to his branch, but now it's Erased timeline instead. Which would mean he wants to take all his forces with him the first time, because he can't get access to them after his first trip to the past.