I see this suggestion quite often. It is really a problem that some people in Tower Defense don't understand how the game works and they keep placing their towers wrong even when you told them not to and this is really frustrating. I think bringing back the votekick command would be the best way to solve this problem.
Solo Tower Defense could become a thing in my opinion tho. It will probably attract enough players to fill games as it only requires 2 players, and I don't think it will pull away too many players from Team Tower Defense as people will still be playing with their friends. With a few adjustments to pricings etc. the game could be good for a 1v1 gamemode.