I feel strongly that 2 cooperative play issues need to be addressed:
1) A co-op player logging in prevents the account from going barbarian. Forever.
2) There is no limit to the amount of players one person can co-op.
These two factors combine to allow one or two tribe leaders to run many accounts at once, as it seems to be common practice for tribe leaders to ask all members to set a leader as co-op in case of inactivity. Over time, inevitably, many players quit, and each dead account can become a multi-account for whoever has the co-op.
I'm all for the unlimited use of the account's abilities on co-op, but after 1 month of no valid password login, co-op should either be dropped, or vills begin to turn barb, or both. And possibly set a limit for the amount of active co-ops. perhaps an unlimited amount of 'requested' co-ops is fine, but the number of 'active' co-ops could be limited to 5 or something. For instance, if somebody had 5 active co-ops and wanted to add one more, they would first have to choose one to drop.