In another thread, IceIX said that they already have the daily rewards keyed out to day 300. However, they are intentionally keeping them hidden so that they can be changed if needed if a problem occurs (Ex: Patch appearing as a daily reward before being officially released; was replaced with Hulk).
What happens after that last level
We don’t know if there’s ever supposed to be an end. As no-one is beyond round about day 150 (since the daily rewards are only that old), having plans up to day 300 may well just be a safety margin on an effectively infinite series of rewards.
(Despite the leap from 3* covers to a 4* here, it seems possible that the series might just drop back and include the many 3* characters who haven’t featured in the rewards yet.)
I get green Grey Widow today, I’m so excited, I’ve never seen one of those covers in the wild and I needz it. Feels a bit weird to rely on a Daily Reward for it but I can live with that.