In addition to the (twice?) mentioned Rifts, there is also Torg (and the more recent Torg Eternity version). Here you have six (maybe more) different worlds that start to overlap when BBEGs from the worlds hatch upon a plot to steal 'possibility energy' from our (Core) Earth. You can have high fantasy adventures (Asyle), the cyberpunk inquisition (Cyberpapacy), jungle princesses riding dinosaurs (Living Land), 1930s pulp action (Nile Empire), post-apocalyptic action/horror (Tharkold), more 'classic' Vicotorian-ish horrors (Orrosh), utramodern conspiracy/Umbrella Corp science-gone-wrong (Pan-Pacifica) and more (smaller, more limited cosms) all with unified mechanics and a weird way that character might adapt to the cosms they visit, or might stay the same.