Two suggestions:
1) Warmode is more of a Mode than a Challenge, but the challenge aspect of it is really cool. If it's kept as both a Mode and Challenge, obviously it can only be completed (for rewards) upon reaching lvl 60 if it was never disabled pre-60. On the other hand, as a Mode it shouldn't be automatically turned off upon relogging (at lvl 60) as it was in TurtleWoW.
2) Would be nice if any challenges could be disabled (but not re-enabled) pre-60. Of course this would remove any rewards (don't know how this would be implemented with S&S that has a continuous reward structure).