I'll just add my voice to Lanefan's original post. I would very much like to see 5e be able to handle groups of 1) various numbers of PCs and 2) variable levels of those PCs within the same party.
. . .
There was never "Hey no fair! They're 5th level and I'm still 4th!" or "How come they get to be 4th but I haven't made 2nd yet! This isn't 'Balanced'!" It was just the way it was...and we had fun doing it.
Nod, I also want to see this. I run two campaigns, in 3.5e.
1) Five characters, levels 2-4. After leveling up when they finish the current adventure, it will be levels 3-5.
2) Nine characters, levels 4-9.
-- 1 9th level human monk, who started at 1st level, and has been played in 3 of my campaigns in the same world
-- 1 8th level human fighter, in this party from the beginning starting at 1st level, party leader and done some stuff on his own
-- 2 7th level, human cleric & human wizard/sorcerer, in this party from the beginning starting at 1st level
-- 2 6th level, human ranger/rogue and half-elf druid, in this party from the beginning starting at 1st level, but actually changed players, as old players left and new players joined and took them over. One of them just needs to train to get 7th, has the XP already.
-- 2 5th level, elf wizard and elf fighter/wizard, joined together after the party was up and running for a while, started at 1st level when the others were 2-3 level. Both have the XP for 6th, but haven't trained yet.
-- 1 4th level human cleric, just joined by taking over an NPC who had helped the PC's on a previous mission. The player got to level up one level from the original module stats for the NPC, for the NPC's "on screen" experience before he joined. Will easily jump to 6th if she survives this adventure.
Nobody complains about the level differences, because every PC EARNED the levels fairly -- no free lunches, except maybe the most junior character, which nobody begrudges (and taking over an NPC with levels and experience seems fair, too).
To deal with this group, I build major set-piece combat encounters so there's a big diverse mix of stuff to fight, appropriate to a wide range of levels (e.g.,2nd level enemy warriors with poison and potions, hell hounds, gargoyles, and a flesh golem in one fight; dire rats, owlbears, and wererats, some with PC levels, in another). It's more dangerous for the low level characters (sometimes they tangle it up with the big bads), but it's exciting and everybody gets a chance to contribute and a chance to be in danger.
There's also a lot of non-combat stuff, where level doesn't really matter. Right now, the PC's are helping fight a fire and rescue people, in between trying to figure out a mystery. Level doesn't seem to affect any of that.
Also, in RP terms, I don't make lower level = less influence or less important. The 4th level NPC turned PC is the daughter of the city's bishop, who is an adviser and old friend of the national ruler -- so she's more "connected" than anyone else. And she's the only PC from the city they're in, so she gets to play "Basil Exposition" in explaining who big NPC's are, what's going on in the city, etc. She and 3 of the 4 high level PC's and the ones who NPC's often recognize and defer to -- not because of their levels, but because of who they are in the setting.