Not that I would want this, but to answer the Topic.
What I would do is sort of a 2 part deal.
Part 1 would be to slightly increase the exp gained from mobs around 1-55 roughly 10% exlcuding TOAU and WOTG areas/mobs.
Also I would add EXP tiers in Abyssea,
30-49 would see a 50% loss in ANY EXP gained, in Abyssea, wether from mobs, or chests.
50-69 would see a 30% loss in ANY EXP gained, in Abyssea, wether from mobs, or chests.
70+ would see no reduction in EXP gain.
This would still allow people -70 to join Abyssea, but reduce the amount of exp gained.
Part 2 is still open, I haven't completely thought this through, but one option could be adding all new low-level specific BCNMS to boost low-level partying.
Allow me to explain.
My idea would be to implement a "key item" that all mobs from levels 1-55(or somewhere around there) that are EP or higher to your current level, randomly drop, similar to seal drops. This "key item" is obtained by everyone in the party when it is dropped, excluding anyone in the party that the mob may be "too weak" too. And also excluding ToAH & WoTG zones. This would create an incentive to level lower level jobs.
Now what do you with the "key items" you say? Well, it's simple.
Imagine all new BCNMs divided by level ranges, each range requiring a certain amount of the "key item" (just like regular BCNM's are).
Range 1. Lv. 11 - 20 to enter.
Range 2. Lv. 21 - 30
Range 3. Lv. 31 - 40
Range 4. Lv. 41 - 50
In each range, possibly 3 to 4 different fights (don't wont to get burnt out on the same fight every time). The difficulty of the fights would vary depending on how many people in group/alliance, the max being an alliance of 12.
Now for the rewards. I'll split them up into different catergories.
EXP: The EXP gained per player would be equal. But the amount of EXP would be determined by how the group performs. In a way similar to Campaign, various EXP bonuses would be given by doing Skillchains, Magic Bursts, Weapon Skills, Job abilities, Spells, Ranged attacks etc, but applying caps so in order to maximize EXP, you must have a diverse group, But also have it so the higher the level range, the higher the base EXP gained, so naturally a level 40 group would get more EXP than a level 20.
GIL: Everyone loves Gil, I would say have a set amount of gil for each level range. 10k for any range 1 BCNM, 20k for any range 2, 30k for any range 3, and 40k for any range 4. (This is just a basic outline for how gil would be obtained, not saying those exact amounts).
LOOT: Of course you would have a treasure chest sitting there at the end. There would be a LARGE loot list, including anything from Traverse Stones, to NM drops (Leaping Boots, Emp hairpin, etc.., with very low droprates of course) and everything in between.
This sounded cool in my head atleast, maybe not so much...