Were it straightforward Universal wouldn't have a problem developing something but if Nintendo is stubbornly wanting a gaming mechanic implemented into the marquee attraction like Mario Kart, then it's an added layer of complexity that could prove cumbersome to work with. Not saying that's the case but depending on what Nintendo wants in its final product as a representation of the IP, some things could be on the back-burner until they figure something out.
Though I've played some games in the past I'm not well-versed in the series as a whole. It may help to think of what game mechanics are present in the series which could translate over to a ride (and interactive mini-games in the land), and that could be a reason why this project seems stuck at the moment; it's not like there's a lack of source material in terms of lands, stories, and characters, and I'm sure Nintendo's and Universal's writers could create something fitting of the IP.
If it turns out Nintendo really wants some kind of gaming aspect inserted into the ride then how Universal would normally approach something like this changes drastically.
It's not feasible but it'd be cool if they brought back the Poseidon's Fury walk-through aspect combined with escape room mechanics and build a dungeon/temple puzzle game. In a ride it could be like a voting system and majority of votes is the path/option you take. Not sure how fun that would actually be though.