Wario's Shipyard was a bit of a reference of sorts to the first two Wario Land games,
The pirate theming, the music track having some minor Wario Land influence,
And a cameo of Bull Wario
It's not obivous like DKC and DK Island, but the track does have a very Wario Land vibe.
And it's probably the closest to a Wario Land based track we have in the Mario Kart series. (Even if it's not directly based on anything from Wario Land.)
If we're lucky, some sort of background/musical reference like in Wario Shipyard. If we're even luckier than that, we'll get a Wario Land themed track, though given Diamond City only appeared in the arcade games, it seems spinoffs aren't high on Nintendo's priority list there. And unfortunately, the chances of a Wario Land character in Mario Kart is virtually nill, regardless of how much Captain Syrup would be neat to see in a Mario sports title.