Tribal is a venerable theme for bottom-up blocks. In fact, the second-ever block to have such a theme as we know them, Onslaught, was tribal. Since creature types are printed right there on the type line, and always have been, but usually are aesthetic, it seems natural to want to give them gameplay value. The problem lies in linearity: If you go full-out for a tribal block then deckbuilding under that block comes down to "pick a creature type and put whatever creatures of that type you can into it." This was a serious problem for the Lorwyn (mini-)block and perhaps has scuttled the idea of tribal as a major block theme for the foreseeable future. Unless, as one person suggested on Mark Rosewater's Tumblr blog, R&D uses "Tribal, but inverted."
When cairnwanderer asked, "the chance of seeing a subtheme that encourages players to AVOID same-creature-type as opposed to ENCOURAGING same-creature-type?" he said:
Those tend to work better as individual cards than a subtheme, but perhaps one day, a world will present itself where it makes sense.
Worrying about making your creature types unique, or at least playing lots of them, leads to some pretty interesting mechanical decisions - think running creatures just because they have one-off creature types. In an extreme example of the kind of flavor that lends itself to this theme, yours truly once designed a world called Gombai for an MTGSalvation card creation game where a spell "has caused all but one member of each species to disappear." Obviously the situation doesn't have to be so extreme: seabutcher said, "I could see this working. Some sort of diversity-themed world. Abilities that take effect if you control a certain number of creature types. Creatures with separate abilities that care about different types. Changelings coming back," while yawgmoth3000 said, "If the idea is to create the feel of a very diverse army, with people of all races coming together, I find that class-based type matters, like Morningtide had, can encourage that."