So just turn siege into a strategus siege type game. Use castle and city siege maps, let players use ladders and siege equipment. Set the timer to like 15-20 minutes and/or 300 tickets on each side (or something like that).
If timer runs out, whoever has more tickets at the end wins (or maybe defenders always win if they hold the castle/city), maybe give the defense less tickets since the attackers would likely be dying more? Or not, and put the onus on the attackers to take the defenders spawns.
You'd also prob need to give like 2 or 3 multipliers for the winner for the longer rounds.