He already finished the game last week.
Read the thread.
I played a bit more after when the patch came out, but the site was down so I couldn't say anything about it. The underground (I've explored like 60-70%) seems to entirely be mining deposits, enemy camps, Yiga camps (you get Yiga vehicle blueprints from them), and hidden chests that give you the DLC armours from Botw, not sure what are new since I ignored a lot of the EX stuff in botw. No shrines and no koroks underground so its really quite barren once you actually light the area up.
I did the Yiga quest to unlock the 'earthquake technique' and its not anything like a TP hidden more, its the shockwave those big Yiga dudes use, and it kinda sucks. You charge your attack with no weapon equipped. Yes the Yiga hole goes to the depths.
I've explored most of the sky give or take a few islands and 2 of the 3 mazes. They all just usually come down to a shrine quest that wants you to do some vehicle thing to complete. There are a new spirit orb thing that seem to only come from the islands, 4 of them to upgrade the attack damage of 1 of the sage companions, not really worth it for how hard they are to find. There are also chests in the sky that give treasure maps that just mark the location of one of the EX armour chests in the depths.
They have gone full collectathon, with a quest to explore every well in the game (the reward is rupees), every cave has one of those big frogs that drop a unique item, but I think its just to track how many you completed because I never found a use for the items. There are a lot of quests that boil down to 'find all of this thing in the world', there is one with cherry trees, wells, specific sky islands that have ancient writing on them. None of these things really get marked when you complete them other than caves, so unless you are using map markers like crazy (which have a limit I think) you will probably lose track.
The house building, which I might have mentioned before or not, is literally assemble your house from room parts using the ultra hand.