I will add more later, but this for me sounds awesome as SM64 and OoT are very formative games for me. I am in the small camp of people that likes OoT's overworld, even Hyrule field.
The real question is, would it be as fun to explore a huge world designed for an entirely different game style? Oot's maps aren't made with platforming or free jumping in mind so it might be boring outside of the concept alone.
Well, like the video shows, Mario does have to take a few workarounds for some situations. For instance, the well in Kakariko becomes a wall-jump challenge to get out of. A few parts of Zora's River looks like it would be passable for platforming with some coins and enemies scattered throughout. To be honest, I can't really imagine anything to fix Hyrule Field. Wing Cap would break the whole exploration angle since you can just fly over places you aren't meant to. I can only think of something like making the roads amp up Mario's running speed three or four times over. That way, the player's launching themselves across the field. It would be fun for novelty's sake, and a decent way of speeding the area up.
Some of the puzzles could be completely reworked, as well. Mario could have to lure a propeller enemy near the Shadow Temple entrance in order to reach it. The entrance to the Water Temple could be set to involve using the Metal Cap to step on an underwater button. Wrap a bit of the Biggoron Sword sequence in there as well, using the high-speed Hyrule Field idea, so you're having to accelerate to Lake Hylia while on a timer, from a Metal Cap box placed somewhere like Lon Lon Ranch. Don't forget replacing the Running Man with Koopa the Quick and a Shiny Shell to ride on.
Skelux is known for pulling off some really neat stuff on a whim. If you've ever seen how Shadow Mario was coded in Sunshine, I'll bet anything that copying the concept into SM64 would be child's play, and would make an interesting Dark Link parallel. Since Link has various tools to help him, Mario's platforming would make a different approach. For instance, Mario doesn't have anything like the Hookshot, and needs a temporary Metal Cap powerup to mimic the Iron Boots. Bombable walls would require finding a Bob-Omb and hauling it to the right spot before it explodes in Mario's hands, although I'd imagine that the timer for that would be slightly extended. Similarly, while Link has to find ways to burn stuff like spider webs, Mario just needs a Minish Cap.
It would take some creative tinkering, and I wouldn't argue against placing a few things like crates and makeshift walls to modify the area and make platforming a bit more prevalent. (Like extending the height for the jump needed to enter the Shadow Temple by turning the railing into a fence, for that propeller enemy idea.) You could do quite a bit with the Ocarina of Time overworld.