First of all, fix the UI. It looks bad; cheap; not great. It looks like what I imagine the GameCube OS would look like if it had not had anything to do with a cube. So what should it actually look like? Well, I'll show you.
The Wii U Menu is well-designed. It can show 15 main things at a time compared to the 3 that the Switch can show. You could say that the Switch only shows 3 at a time because it has to work on a tablet, but all 15 things show on the Wii U GamePad just fine, no problem. The Switch uses the same screen size with a higher resolution; the Switch should at least show 6 at a time.
Another thing that needs to be fixed is similar, but not the same. The menu needs themes. The way it is, the 2
Another thing that needs refining is the eShop. It's near impossible to find anything in it at this point. It has a search system, but the games themselves are categorized poorly. I feel that the Wii Shop Channel categorized things well; It divided things into Virtual Console games, WiiWare games, and apps. So what the eShop should do now is to divide it into Virtual Console games like the NEO GEO AES and Arcade games, eShop exclusives such as the Jackbox Party Pack and Sonic Mania, and retail games such as Arms and Super Bomberman R. Apps would be added later. Then each of those categories are divided by genre, release date, and company. If it was divided like this, I'm sure it would be much easier to use.
Finally, fix the friend system. Replace friend codes with usernames and add a Discord system. The friend codes are too long and randomized for it to be easy to remember. You'd have to have it written down somewhere. I can say "Add me on Xbox Live: Storm9CP" much easier than I can say "Add me on Switch: SW-1802837501918723905 or whatever" From what I hear, things like Netflix and YouTube and Discord are all ready to be launched and Nintendo just has to allow it. So why doesn't Nintendo just allow people to use Discord on the console instead of their dumb mobile phone app?
Going on a bit of a tangent with the mobile phone app, most people complain about it draining their battery. That's fair, but I'm more concerned that it's using more bandwidth. Is it? I have no idea. But if it is, that's no good.
Anyway, that's about all I have for now. Goodbye!