Why does the amount of Missions decrease substantially when all the active followers are level 100?
We were hoping not to have a bunch of irrelevant missions spamming up your mission list. We didn't plan well enough for the fact that those missions aren't really irrelevant when the salvage yard is OP. It's definitely an issue we talk about here, but solutions are complex and can have a lot of ripple-effect consequences on the rest of the system/game.
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Originally missions all pulled from a single pool; players found that as they got their first level 100 followers they got fewer low-level missions and had a harder time leveling their weaker followers, so we made a change to ensure that some low-level missions would always pop if you had a follower in that range who needed them. We recognize this causes some odd incentives (you're encouraged to NOT max all your followers) and are discussing good solutions to that problem.