One well-known bottom-up (that is, mechanics-first) block theme that has been done multiple times is tribal/creature types matter. The two times it was a major theme got mixed responses: Almost everyone loved Onslaught block, but not so much Lorwyn block. The explanation of it is rather Goldilocks-esque: Lorwyn was the Papa Bear with too much tribal, Onslaught the Baby Bear with the just-right amount.
Mark Rosewater has explained as such on his Tumblr blog. When iamatimetravelerfrom2036ad asked, "Why was tribal well executed in Onslaught but not in Lorwyn?" Rosewater responded:
I think we turned up the volume a little too loud in Lorwyn. It was fine for constructed but hurt limited especially draft.
However, that doesn't mean R&D is going to play it entirely close to the vest with tribal from now on. When dannyg28 asked, "Is Innistrad close to the most tribal we'll see in a set going forward post-lowryn or do you think if a set's primary focus was tribal you could afford to do a bit more?" Rosewater said:
I believe we could have a block with a little more tribal than Innistrad but not as much as Lorwyn.
In other words, an entirely new tribal plane may be in the works in the near future: Perhaps a massive gladiatorial arena where creature types are organized into fighting schools, or else a primitive and harsh world where loners die and the survivors stick together.