I was just wondering if there's a way to make platforms (GameCube, Xbox, etc.) be subcategories. If it's not, that's a feature I would like to see.

cant you just rename the variables as Gamecube/Xbox/etc?

Yes. but then there would be two variable tags saying what platform you are on, and even if you removed platforms and replaced them with variables, you lose the platform tags. It would also be a bit confusing to new runners. I think that just a little tick box in the Edit Game section to make platforms subcategories would be a better solution.

And if they do add that, I think it would be good to make it so that you can lump platforms into one subcategory so that DS and 3DS or something like that wouldn't be separate subcategories.

For now I think double variables is the best that can be managed. Making platforms able to be subcategories but then allowing arbitrary groupings of them utilized as subcategories isn't a trivial amount of work.