Resources that come to mind: 1, 2

A standardized tileset layout would have some severe limitations, I think. Diamondus, for instance, has six different basic floor tiles; Castle has three; Tubelectric has one. Rearranging their layouts isn't going to help you switch between them there. Or look at something like Graesy... it has a massive number of different connection tiles at the top that most tilesets don't have any analogs for. And that's without getting into trying to swap between, say, Diamondus, where floors are only one tile high; Jungle, where they're two; and Colonius, where they're three.