I think it would be really awesome if itch.io did more with not only the game developers, but the game engine creators as well. Starting today Construct 2 is now available on itch.io (and it's 30% off until next Monday). You can download the trial, and if you decide to upgrade, purchase a registration key right through itch.io like you would any game.
Check it out: http://scirra.itch.io/construct-2-game-maker-tool
Also, if there are any other engines that you think might fit on itch.io tell us.