First, shinies. Best shinies, hands down, are available to heroes. Getting shinies at lower levels generally involved following heroes around and jacking from their corpses. However, this comes with its own stigma, and players like me think its really lame if thats all my character really does. I've noticed that when my characters get access to more powerful gear early on, I'm more prone to go explore or pk with him before hero. If my guy is still using ragesteel, skull rings, and charred bracers at level 40, I figure I might as well hit hero in order to grab the best gear possible, rather than waste my time gathering shinies that I'll end up replacing later anyway, or pking at a disadvantage versus full hero sets.

I think creating powerful limited items that become significantly less effective at hero would be a good way to remedy the situation. You could also use these items to remedy extreme imbalances, rather than tweaking the nature of the classes themselves.

For example, a pair of magical boots which allow elf/dark elf thieves under level 40 to reflect deathblows, but crumble when worn by someone past level 40. No big balance killer, just something which makes playing a gimped class like elf thief more bearable in the middle ranks.

Or a prep which you eat that nullifies the first bash that hits you and returns ice damage. However, the second and third bashes will hit and their effects will not be nullified. This sort of thing would help nullify the giant powerbasher combos, and encourage more mages and bards and people who are really vulnerable to bash to stick around, as they might actually have a chance versus that stuff.

Things like that which make me think I have a fighting chance~

Second, cabal balance. Certain cabals are significantly, significantly stronger at lower levels than others, which don't reach their peak until hero.

Tribunal, Outlander, Battle, and Nexus have significant powers at lower levels. If you make a chart of some of the best lowbie pkers in the game, I'd suggest that of the caballed ones, the majority of them skew towards outlander and battle.

Tribunal and Nexus can still both contribute significantly at lower levels, though their powers are less dangerous in pk. But you can still explore very well with nexus powers, for example.

Scion and Empire have decent powers at lower levels, but really their powers generally don't reach the peak until hero. However, with so many enemies they generally have a more difficult time leveling than other cabals, so they can take awhile to hit hero anyway.

Fortress powers are just trash at low levels, and promotion is based on imm approval. Thus, fort characters have a huge incentive to charge straight to hero, as Imms generally snoop long-lasting hero characters more than lowbie-midbie characters (for good reason!). Thus, you're much more likely to be promoted and given access to powers that aren't such trash at hero. There do exist cases of midbie maran/acolyte, but those are generally given to exceptional players. Its hard to justify maraning a good midbie whose at level 38 with 80 hours, when you have a bunch of hero squires who aren't quite as good, but have heart and have waited much longer and tried just as hard.