I believe over the years they made leveling considerably easier, I remember back in the day it used to take a few months to even reach level 100. It would be nice if there was a Hellmode setting that you could talk to the fairy on birth island and set and in increases the exp. needed to level, or reduced the exp. gained from mobs for the character to some fraction of what's normal, so leveling speed will be roughly in line with what it originally was. Then if you hit the level cap you get a title or achievement or something that doesn't affect gameplay.
Also if you get bored of hell mode you can talk to like a mayor or any of the towns and leave it, then either....
you just leave hellmode you level stays the same, future leveling becomes easier
the game converts the exp you've accumulated so far in hellmode and converts it to a regular level and you level up a whole bunch all at once
Then once you leave hellmode you can't enter it again, and only can't get the title/achievement.
A few notes
It's fine to allow buying/trading of items unlike runescape ironman mode, as that was always a part of classic rose, plus it'll add value to a bunch of low level sets that people don't much use because they out level them to quickly.
Should places like Desert of the Dead and Clan Ground where you can get huge amounts of exp. that were not in the game back them be disabled while in hellmode?