"target land becomes an island" is really, really boring. So lets put it as a rider on other spells, so it comes up sometimes and may matter, either because you coincidentally have islandwalk creatures, or because your opponent taps their mana wrong in response.

Also, we need more counterspell variants :)

Nifty idea.

Thx

Sorta nice, but unless they have a lot of colourless stuff, they should just respond to it by tapping their lands before they become islands - which seems unfun.

True, but that's still inconvenient: it doesn't help if it happens during combat but they wanted to play creatures after, for instance.