...which means you need to have different concepts for single and multiplayer. Clear?
Good design is possible in both. Multiplayer needs are not an excuse for a design that gives a poor singleplayer experience. If you believe that there is no way to have both a good design and multiplayer, that's really sad. Perhaps you can suggest some eloquent solutions as opposed to being a negatron?
Which means that you CAN NOT HAVE different concepts for single and multiplayer, since this game has a drop-in-drop-out feature.
Anything that works in single-player HAS to work in multiplayer, and vice versa, given that a single-player game can become a multiplayer one at any moment, then return single-player and so on, so forth.
In light of the above, any hypothetical improvement to the UI has to be intended for both kind of experiences.