I would like to suggest an endurance raid. You could simply link existing pve levels together to form a long string. For example, players use their existing hero pool to begin battling through pve levels. As they complete levels, they climb higher up the difficulty ladder. They can keep fighting until all of their heroes are exhausted.
Award gems based upon heroes that survive each wave; sort of the inverse of pvp matches. After three waves, a random award chest is unlocked.
This would add a layer of pve to the game for those who don’t like to pvp, but want to play more than stamina allows. They would also receive minor rewards similar to the pvp chests. The raid would almost perfectly mirror the pvp matches in awards and length of play time and would really be a boon to your pve’rs.
You could also allow gold purchases to reset dead heroes in this as well, just like in pvp. The endurance raid could reset the same time as the pvp reset. Most people won’t have time to do both pvp and the endurance raid to completion, so the influx of rewards shouldn’t upset the game. I would organize it so that the frag rewards of heroes more useful in pve drop in the endurance raid chests and the pvp oriented hero frags drop in pvp chests.
Thanks and great job with the game.