Personally, I'd like to see a DX version of the original Wario Land-I was always bummed it didn't have SGB support having been released a few months before the peripheral. Other ideas:
-Game Boy Printer support (several of the DX games added this-sometimes it was just digitized art, but it would be thematically appropriate)
-Play as Waluigi (seeing as Luigi was added to SML2 DX). Maybe give him a different objective for what he wants with the money.
-More levels with treasures
-Extended ending where
-Better way to get enough coins for the best ending without having to grind or abuse the money minigame.
-More secret exits (I was always disappointed that the last three worlds don't have one). One that acts as a shortcut in Parsley Woods might be good.
-More places where the Bull and Dragon hats have advantages over the Jet hat.
Some of these are obviously (much) more difficult than others, but I figured I would throw everything out there.