Imagine a cosmos full of those floating islands I've posted about previously. Each island harbors a single fantasy race. They float or orbit relatively close to each other. Spelljammer type ships or chariots pulled by geese or whatever will allow the races to travel between them.
Alignments are out or at the very least not mandated according to race. An orc can be of good alignment (though it's better to be safe and hang him - old elven proverb.) Unlike the typical fantasy setting the races are not necessarily at each others throats but affiliated in a confederation of sorts out of need.
A demonic empire (or maybe a necromantic empire whatever anti-life/anti-freedom trope works for you) has attacked all the islands. Several islands were destroyed in the past. The remainder have begun talks to band together and defeat this evil force once and for all. A prosperous merchant and head of the largest trade guild who has already contributed arms and funds to the war effort has sent his agents to recruit the youngest, bravest and most able specimens of each race (with priority given to those with unusual abilities.) They will form a force to inspire the citizens of the confederation and prove they can all work together (well maybe not the goblins): the Legion of Fantastic Heroes.
One part Legion of Super Heroes, one part X-Men and a huge dollop of your favorite fantasy authors. Maybe they were inspired by the exploits of a godlike survivor of a destroyed island. Perhaps the merchant gets his idea when a psionic, a mage specializing in lightning spells and another with metalworking spells saves him from assassins.
Most of the worlds from the Legion of Super Heroes made little scientific sense but could be transformed into magical islands with a little work. Ditto for the fantastic places in the Marvel Universe Wakanda just screams for a steampunk and savage jungle adventure. In addition to being divided by races islands could be given over (mostly) to the different classes. Planet of Hats meets Island of Rangers!