Recruitment and wars will help the alliance a lot. So, any steps towards those objectives will help.
I would agree with
Max_imus - everyone plays differently. Based on my experience from playing on three bases, I would say maxing everything isn't efficient, and isn't practical unless someone is willing to spend loads of money and time into the game. But, again, different strokes for different folks, so, I don't wish to get into an argument about this here
Base design discussions are a way to keep the chatter going... So are nation and wonder discussions... And, what to upgrade is quite engaging too...
Great suggestion from
Vixen too - it's a strategy game, so, talking strategy is nice
Helping those who are new to the game with warm welcome messages, and answering their questions about the game will help too.
Identifying allies who are inactive and steps to maybe get them to engage or steps towards moving them out of the alliance will also help. An active alliance keeps more active players - if the proportion of inactive players increase, you risk losing the active ones from the alliance.
Troop donations help too - quick fulfillment of troop requests are a nice way to keep the active players. Encouraging everyone to donate will also help.
Cheers & good luck