I haven't completely thought through exactly how this would, or even could, work, but rather just floating an idea.
Just to get the conversation started, let's say attackers could only siege strongholds at minus 5 their level or higher. It might add a new dynamic to the game. Houses would want the strongest players at each threshold. It would make the game more team oriented and make less powerful players feel like they are actually contributing more to their house.
I think it would change the basic linear, upgrade-as-fast-as-possible nature of the game. Players would spend more time at the top of each tier, upgrading troops, buildings, research, etc ... rather than upgrade stronghold level at the cost of just about everything else.
Given how long upgrade times are in the later stages of the game, this might make the game more appealing to a wider base of players, especially if the creators of the game raise the current stronghold limit of 40.
I'm sure there are some problems with this that would need to be addressed, but it very well could help extend the longevity of the game and make progression more strategic than it is now.