[Tuor of Gondolin]

The most interesting "alternate history" scenario would have
been the Noldor and elf allies returning to
Middle-earth without the Curse of Mandos, as was clearly possible. It
was clearly stated (in The Silmarillion?) that
there was no bar per se to elves leaving Valinor. Without a Curse
it is conceivable that a "balance of power"
might have evolved in Beleriand and that (like in the 1940s-1980s
Cold War) the conflict between elves/Edain and
Morgoth might have moved to the Third World of the rest of
Middle-earth, rather like the standoff in Europe
between NATO and the Warsaw Pact, in which case you could
have the houses of the dwarves and men
as much sought after allies by both sides.