For that matter its as simple as the weird utility lands like Mistveil Plains - just print a land with one subtype and the requisite text.

Baseball Field
Land - Plains

(T: add W to your mana pool.)
T: add D to your mana pool.

The problem with this is it kind of fucks with the idea that spells with a D in the cost are uniquely costed. If you can fetch a UD dual because it's an Island, then it's no different than a regularly costed spell.

I mean they COULD do this and maybe you'd see decks crop up that have easy access to D, and they would have rad manabase names like BUD Combro or DRUG Prison or something. Wouldn't that be some shit.

If they don't do this though, it actually kind of makes the old painlands a little better because they would effectively tap for three distinct kinds of mana.