I think it would be wonderful to allow guild members to be able to set their own guild background. I love the guild leader, but our background changes only once... ever. We have more options, so I want to see them!
Secondly, yes, make use of an assignable officer with guild leader privileges. It would take pressure off of the leader while not just giving it to the people with the most bp. That said, I dominate my guild's bp but would not give myself this power, as I'd brawl all day, and win many, but it'd be insane for other members.
All in all, love the casual guild experience, but don't know about making it too hardcore. It burns people out, and we'll lose too many players if the top guilds just destroy everyone as much as possible to maximize their win/loss record or whatever. Maybe more diminishing returns while still allowing some spamming, and then make sure guilds don't fight the same guild over once a day or few days.
That's my two pence!