Adjust the progress missions/arena challenges in relationship to the difficulty it is to advance in the arena.

Please. With sugar on top.

Thanks

You're only about the 5000th poster to suggest that, or something similar. The odds of it actually happening may be better now than they were a couple thousand posts ago, but they're still not very good. Bluntly put, the vast majority of players should just forget about ever getting Arbiter from progress missions - at best, you might get her from progress missions about six months after pulling her and a bunch of other S tier legendaries from shards, at the point where you don't need her in the slightest anymore.