As for me, I'm thinking it's most likely Simon will get immunity from other people's powers (or super agility, or keep his invisibility, which I'm kind of rooting for).

I think that the only way Simon wouldn't take his invisibility back is if the power was unavailable. The way I see it, he not only feels that his power is a part of him, but I think he feels that he's deviated from what was supposed to happen had he kept his invisibility. I think he feels that he's supposed to have this particular power, so I would find it at least a little odd if he opted for another one. Not that I'm completely opposed to the idea of him getting a new power. It would explain why future!Simon never used invisibility, which has kind of baffled me since the reveal. In that case, super agility certainly makes sense.

I had the image of Alisha asking Simon what powers she should get, and in that case I think he'd suggest invulnerability

I can so see that happening. Who better to ask than genre-savvy Simon?

Also, do you have any ideas about what power Kelly might get? I'm quite curious about her as well since I doubt that she'll go back to being a mind reader.