How about the simple idea of having the banner balance only work if the numbers of (same) banners to balance would be about equal for both teams?
Or, if people insist to have banner balance, make the player cost for additional clan members in your team grow exponentially. The first five clan members are "for free", the next one takes 2 player slots, the seventh takes 3, the 8th takes 4, the 9th 5 and the 10th takes 6. With the 17 Greys of the first screen it would mean they would have the value of 88 randomers, if I didn't calculate wrong. Which means they would represent the whole team and everyone else would be fighting them.
In my eyes clans are meant a) for strategus or b) to play together with friends, and NOT to milk pubs by default. Sharing the banner with a lot of other people should not make the game easier, automatically.