Please let me reassure you. In my ‘World of’ MMO countdown, I did not forget about mechs. There are obviously a lot of games out there that implement mechs to varying success. So a game that uses just mechs wouldn't necessarily be interesting.
Then few days ago, I stumbled upon Pacific Rim again, and it was enlightenment!
There is nothing wrong with mechs. They are not boring by any means. But to make them unique, you have to put them in interesting situations. This is exactly what Pacific Rim does by introducing Jaegers fighting against Kaiju.
This creates more possibilities for rather interesting game design. On the one side, we could have Jaegers, huge mechwarrior machines with two pilots. Those could easily use the economy and leveling up from World of Tanks.
On the other side, Kaiju, genetically engineered monsters fighting with the use of, let’s call it – nature. They would need a slightly different system of gaining experience and skills. This could make the game even more interesting and full of variety.
Creatures and machines in World of Jaegers and Kaiju have no limits. Designs could be endless, going way above Tier 10.