Is there any chance we could get some sort of consolidated/combined buff between the mercenary battalion seasons? As it is currently implemented, any mercenary unlocked and leveled prior to the most current expansion will not give or contribute to the newest buff, making them pretty much useless. This disincentivizes spending DBC on any more than 1 mercenary unlock per season/year, as they would all eventually become useless the next year. Consider implementing some method of making past mercenaries give and contribute to current battalion buffs. One way to do this would be to make it so once a mercenary from the newest battalion buff season (Reign of Shadows, for example) is unlocked and reaches level 5, previous season mercenaries will switch from their previous season buff (Shattered Lands or BoL) to the most current buff (Reign of Shadows). However, because they are from the previous season, they can be made to not contribute as much to the RoS buff as much as RoS mercenaries do. The amount they contribute could be made similar to how much xp previous season familiars give and require to level. Overall, the idea is to make past mercenaries continuously useful. There are a lot of great looking mercenaries in the DBC store that are essentially not worth buying because their buff is relatively very weak. Implementing the suggestions above or something similar would encourage players to purchase/unlock more mercenaries and continue using the mercenary they feel is best for their character (Make Raffik Great Again!).
