A big problem I see with this is that it only adds stat changes to enemies. What you’re proposing is similar to the Master mode in Terraria. For comparison, Terraria has three difficulty modes. Normal, the vanilla experience, Expert, the medium difficulty, and Master, the hardest one. What Expert mode does better than Master mode in Terraria is that it not only raises enemy stats, but also adds new attacks, which add more variety to the difficulty, and unique rewards for doing so. Meanwhile, Master Mode adds almost nothing asides from pets, whereas Expert mode exclusive loot had practical use. What I’m saying is, if we were to add some sort of harder version of RotMG for veterans, it would have to be fresh enough for it to be interesting. It’s one thing to just raise stats and call it a mode, but it’s another to add some content unique to that mode. What if we had more crippling debuffs, or new boss phases in endgame dungeons? I’d want this idea to take the approach of the Hard Mode version of The Shatters, adding in more variety with boss fights but also rewarding the player with that difficulty’s exclusive loot, in this case, the Chrysalis of Eternity.
What I am trying to get at is that simply raising stats isn’t enough, we also have to make the dungeons more varied in this mode to give more incentive to do its harder version. There wouldn’t be much fun for the average player in fighting the Void Entity but with doubled stats, only for a second chance at Omnipotence Ring. However, it would be interesting to see new attacks and enemies, maybe even an entirely new and harder fight, which not only gives higher chances at whites, but has a unique drop, whether it be a buffed version of a preexisting Void UT or an entirely new one.