http://us.battle.net/d3/en-us/forum/topic/10894620041#6
Replying to someones post about allowing more challenging difficulty in the PTR.
The systems patch when it comes to vanilla Diablo III, is not really meant for well geared level 60 characters (at least, not with the current tuning of Master). It doesn’t help with dynamic difficulty that monsters also max out at level 60 (before, they’d be 63 with MP enabled). Thus the current difficulty tuning at the high end represents a huge nerf.
True, while a level 70 RoS player can create a Master difficulty game, and invite a non-RoS player to it (thus creating a difficulty that is much more in-line for better geared vanilla players), I do not know if that is intended behavior; it could be a co-op bug because while monster levels are based on the host’s character level, co-op games that include both RoS and non-RoS players are supposed to revert to vanilla status (meaning that even monster level and thus the games difficulty, is supposed to revert to vanilla levels; only the high level character is supposed to remain OP within that instance).
Therefore, what the devs really need to re-think is adding more granular control for Master difficulty (like in the console and how Torment is) with incremental settings (like Master I through IV) for vanilla Diablo III. Master IV for example would be tuned to represent the kind of difficulty one would expect when playing a level 67-69 character on Master difficulty in the expansion. These incremental settings for Master would only apply to vanilla and would disappear in the expansion (since Torment has this sort of incremental setting). I believe some implementation like this would be a fair compromise such that there is some sembelance of challenge/difficulty afforded to vanilla players (since not everyone is planning to upgrade).