Mods are what kept the game alive for me. There's only a few things I would change:
1) make set bonuses stronger. A good secondary is stronger than a set bonus which I think is kind of absurd. I don't think it's worth it to farm an entire first order squad for the 10% offense bonus
2) increase the drop rate from challenges. There's too many other rng factors involved in the mod you get. The last thing I want to deal with is doing four battles and not landing a single mod (or landing one 3* gray defense mod, which might as well be the same thing). Would it really kill them to do double drop days once a month?