As we have been doing, add extra threat at start and each round - still have to find the right balance of initial threat and threat per round.
The second idea is to give the imperial player a second class, along with a way to earn extra experience. Possibly +1 xp for every two heroes wounded. The down side is that there is already a lot to manage as the imperial player, so it becomes a major book keeping issue.
The third option is to use the skirmish initiative token. At the start of the initial rebel deployment, the imperial player assigns that to a rebel player. That rebel player may only take one action that round. At the end of that round, the token is given to the rebel player to their left.
Since it seems most of the rebel heroes have free movement options, it can hurt, but is a reasonably fair limit the five players but just penalize one person.
Next option is give the imperial player the initiative token during the initial deployment and all imperial deployment phases. This token can be spent in one of three ways: the token can assigned to a deployment card. Each figure on this deployment card gets +1 action. This action allows the imperial to attack more than once, however, any given action can only be done once. This makes it more useful for certain deployment cards, such as an officer, where that figure could move, attack, and then command. Second use, the token can be spent to activate two imperial deployment cards consecutively, effectively as an interrupt to a rebel player. Third option is to spend the token to get a number of threat equal to the number of rebel players. This is my favourite of the ideas I present, as it is minimal book keeping, and adds some versatility to the imperial player.
The final idea, is that wounded heroes can only do one action. Possibly coupling that with. when they wound a rebel player, the imperial player either assigns the hero a bleed or stun token.