I don't do much reaper myself, but I've seen a lot of people wanting harder reaper difficulty, including some ideas that seem more of a joke.
SO here is my baseline idea for reaper difficulty, and yes it is intended that no one actually plays R10, and that if anyone ever does the devs can use that as a sign they screwed up somewhere.
So, the idea is that since reaper is for every quest, to basically make it so each skull of reaper is +5 levels to the quest lvl. So a lvl 1 quest on reaper 2 is now lvl 11. Obviously, r10 is +50 lvls, and if people are beating quests at that point, then something is wrong. Partly, this lets reaper be enjoyed a little bit while still at heroic levels since players can't normally do quests more than their lvl+4.
That is just the baseline, and all the lvl based stuff scales accordingly, such as save DCs and such.
Additionally, Each skull of reaper adds 5 resistance to everything including DR 5/-. This stacks with any existing resistances. If a creature has a weakness, this resistance is halved (for double dmg weakness) or cut by 2/3s (for triple dmg weakness) and applied after the dmg is multiplied.
Enemy spell dmg has the max CL (caster level) cap increased by 5 for each reaper skull. Casters above level 50 no longer suffer such CL caps.
Higher levels are also more likely to have special senses, such as scent (not sure if this is actually in game or not), see invisibility, and true sight.
HP is increased logarithmic increasing multiplier.
Reaper skulls * 20% chance to have the champion types added (like skyborn and such) in addition to the hard difficulty chances, and can even have two types added and stacking (at first this the monsters that would normally have champion plus also gaining title from reaper mode). Eventually every monster is a champion (at reaper 5), and at reaper 10, every monster has two champion buffs, with bosses having two tier 3champion titles. (obviously, some titles are incompatible, such as chaos and order themes).
Additionally, in reaper mode, all rez shrines are removed and players can only come back to life if party members use spells (or cakes). There is a limit to the number of times a player can be rezzed equal to 10-the number of skulls, which means that in r10, no rezzing at all. Additionally, the death penalties last for the entire duration in the quest and do not get removed until leaving the quest.
Well, these are just some basic ideas I'd start off with. What do you guys think?