If you want better control of game difficulty:
Figure out a system of determining character CR (taking into account the effect of equiptment, charcter building skill, TR'ing and general character optimisation) and indexing it to monster CR. I don't know, might take a re-imagining of the whole concept of CR. The resulting tools might be worth the trouble.
Then take that index, re-spec elite quests of a givin level to meet the most improbably (but possible, and thereore inevitable... ) overpowered characters that can be created at that level. Do not scale for number of party members.
respec normal to meet the CR of whatever a fresh faced follower of the chargen's path might be. Do not scale for number of party members.
respec hard to meet the CR of whatever is halfway between a newb and a sociopathically overfocused ...I mean, the improbable (but inevetable) maximum CR character for that level. Do not scale for number of party members.
Make sure every quest, maybe even raids, has a casual setting. Scale that for number of party members.
Epic is a whole different animal, as far as I'm concerned, and not included in my discussion here.
Give lots of xp for elite and hard, to offset the losses people will have from dieing but completeing anyway (which many of you may not know, but is something peope actually used to do).
Lots if xp. To offset dieing and completing anyway. To offset running back from your bind point in the Restless Isles...
Now, if GROUPING is something you'd like to encourage.... I'd suggest (in addition to carefully typed out suggestions above) making grouping worth while in the loot table, for example:
1 person party: base -2
2 or 3 person party: base -1
4 person party: base
5 person party: base +1
6 person party: base +2
WAIT! you say... why link loot to party size but not link party size to difficulty? Because running with a party is its own difficulty setting, really, thats why.