I think it's safe to say that the most universally loved feature of Let's Go was the following/riding Pokémon. I loved too, but if it does come back, I think that there need to be some tweaks made to the system, in order to make it more convenient.
For me, at least, there were two major annoyances with the system. The first was how riding a Pokémon in the water works. Yes, I would rather ride my Gyarados than a surfboard. Who wouldn't? What I do not want to have to do is go to the menu every time I want to get in the water to switch my Gyarados to be the Pokémon following me. Since I have to do this, we have essentially reverted back to the days where you have to use Surf from the party menu, instead of just pressing A in front of water. Clearly it's not a game breaking annoyance, but it would be so much nicer if didn't have to change what Pokémon is following me every time I go in and out of water.
The other big issue is the post-game high flying mechanic. I love the feeling of flying high above everything on my Charizard and Dragonite, but this mechanic was even more inconvenient than the surfing one. Now, if I ever want to go inside, battle, or talk to someone, I have to change my following Pokémon, only to change it back once I'm done. It's annoying to say the least. the only real advantage of high flying I can see is to fly over obstacles, to allow for faster travel, but this becomes a problem when you can't fly over route gates. Suddenly fast travel over more than one route at a time becomes much more tedious, as you have to get off and on your Pokémon, defeating the entire purpose of flying high. This mechanic would have worked much better if it was optional.
Both problems could have easily been fixed in multiple ways. I think the best way would have been to have a button that toggles through your team for which one follows you. So let's say I'm riding my Charizard, but need to go inside, all I would need to do is a press a button, and toggle over to me Gengar, so I'll land on th ground without having to access any menus. Then, once I'm done, I just press the button a few more times to toggle back to my Charizard. Or if I'm standing at water, all I have to do is toggle to my Gyarados, and then I can hop right in. So much easier. Even easier if it's two buttons (let's say the L and R buttons), which allow you to toggle forward and back. If following/riding Pokémon return in Gen VIII, I truly hope that they include this solution.