However, I can see a game with a curve that's that is flatter than regular D&D. I'm not a fan of something like E6 which has one set of rules for early game and another, different, set of advancement rules after a certain point. You could just as easily adjust the leveling charts to account for limited hit point growth, for example, without actually stopping level advancement.
Old school D&D already has something similar to E6. Hit point progression doesn't stop at name level, but it does flatten out. A Basic fighter, for instance, gets 1d8+Con bonus/level up to 9th level, then +2 hp/level after. XP requirements to gain a new level also shift from exponential (x2/level) to linear (+X per level). The main difference is a Basic fighter continues to advance in to hit rolls and saves, and if used, skills and weapon mastery. And spellcasters still gain new spell levels (up to 16th to 21st level) and spells.
In contrast, E6 characters stop gaining hp, BAB, base save bonuses, and just gain new feats (which can be used to gain new skill levels and spells, but not to exceed the spell or skill level caps). XP requirements also shift from triangular to linear (+X/3 per new feat).
So E6 shifts to a kind of lateral progression at 6th level, where improvements in the primary game-stats are capped (with some minor exceptions), and characters are restricted to adding new abilities rather than improving existing ones. In contrast, the old school version only really caps hit points. Which is odd, because things like fireball damage still increase, which ends up hurting warriors and helping spellcasters (who really don't need it, at that point). Both BECMI and AD&D2e tried to deal with this by adding damage caps to at least some spells (20d in BECMI, variable but often 10d in 2e).
I find E6 to be more coherent. It really is a cap, while the name level cap in old school D&D is mostly about XP and hit points. Extending the concept, an E9 version of Basic D&D would cap saves, the to hit tables, spell levels (5th), and probably cap the maximum level of weapon mastery (Expert? Grand Master?), but allow new weapon mastery, skill, and spell slots. Have to play with the XP costs in BECMI E9, but each new feat after 6th level in E6 costs 1/3rd the number of XP required to reach 6th level. Charging say 60K for new skill slots, 60K for a new weapon mastery slot for fighters and 120K for everyone else, and 50/75/100K for new spell slots for clerics/magic-users/elves might be reasonable.