At a presentation ATM but just want to chime in
Having every world be its own thread, controlled by its GM, and linked to the central hub thread sounds like the best idea. Having a single world with GMs operating different missions/events has already been done, I think we should try for something new.
I don't think the "Person becomes an expy of their former self when they enter a world with different worlds" would be a good idea. People crossing between worlds may as well be native amnesia patients with odd skills (and it kind of enforces a "gamey" feel to each world). Perhaps, as an alternate to turning cyborgs into more physically fit cavemen with sharper spears when they enter a caveman world, we could move the mechanism of balancing to within the hub(s), perhaps some police entity tasked with preventing inter-world conflicts. Additionally, compensating for tech difference (and not just magic) is really gamey, since tech for the most parts relies on physics, which are likely to be constant between most worlds.
There are advantages and disadvantages to both "Magic works the same as in its native world" and "Magic people use the magic of whatever world they're currently in." For the former, you can have as a plot element people trying to exploit the powers of other worlds to achieve something in their own, or someone from one world mounting an invasion, or someone going around helping in small adventures by doing things natives cannot. For the latter, you might have people using other worlds as a solution by bringing their enemy to one where the advantage of their native power does not work.