With regards to co-playing and it's perceived activity advantages, would an automatically enforced 'sleep mode' help balance things?... i.e. if an account had to take an enforced break of (for example) 6 hours every 24 hours then could it take away some of the advantages that a co-played account currently could have over a solo played account.
Such a thing already exists in speed rounds and works quite well... in my experience an account on worlds where a sleep mode is present has the option to select a sleep period of between 6 and 10 hours (users preference), also it can start at a time of your choosing, which would possibly be welcomed by those who have less time to play or who have issues with the time that night bonus finishes in a particular world. (By the way, if you fail to select a sleep mode period yourself then it automatically kicks in after a set amount of time).
I am not sure how well it would work on a regular speed world, so it's just a suggestion.
(With regards to the potential argument that people who are under attack could enter sleep mode, the sleep mode has a delay so cannot be selected to start immediately, plus building troops and construction etc are paused during sleep mode, so there is no advantage to be gained whilst sleep mode is activated, because as soon as sleep mode is over the player will be in exactly the same position as when their sleep mode started).
Personally, the biggest issue I find with others co-playing is (as already mentioned by other players above) the issue of merging accounts. To me it is so frustrating that 'pushing' is supposed to be against the rules and yet one account can hand their villages to another account (to co-play) without penalty. I have actually stopped playing regular worlds because of this merging and I personally just play speed rounds now. It seems a common tactic in regular worlds that groups of players now begin such worlds separately and then merge their accounts into one larger account, whether it was their intention at the start or not is irrelevant, because merging accounts and gifting villages offers accounts advantages which ruin the competitiveness of the game for players who play solo.