So I've been thinking about the thief since we decided to go ahead and adopt BECM/Rules Cyclopedia as our core rules set for the Rainy City campaign.
Because they suffer a bit in the 1-36 level spread. They just don't get truly competent at the speed they did in earlier versions of the game, and while I see the rationale for it (at least, I get where that comes from), I'd be happier with faster advancement. Balance between classes is only a minor issue for me in D&D, and I'm more than happy to let wizards become powerful in ways mere mortals can't even approach. That said, wizards (and clerics) just get even higher level spells compared to B/X, while thieves get a slowed down Thief skill progression.
I briefly considered just going with the B/X progression, but that interferes with my ability to actually rely on the BECM/RC rules as a simple core reference. (And yes, this is a bit of an odd concern given that I'm happily using the 1e and 2e AD&D Monster Manuals and 2e Wizard's Spell Compendium and Ecyclopedia Magica books in the campaign, but there it is.)
Instead, I'm planning on taking the rules for picking pockets and expanding them to the rest of the thief skills. The rule for pick pockets is basically this -- if you're picking the pocket of a higher level character, subtract 5% per difference in level. In the Rules Cyclopedia, this is a one way street -- you don't get any benefits if you're higher level than the victim.
I'm thinking about a) making this work in both directions and b) applying it to any thief skill it seems to fit. So if you're 5th level and picking the pocket of a 2nd level victim, you'd get a +15% to your chance of success. Similarly, if you're hiding from a lower level opponent, you'd get the appropriate bonus. This makes the skills a little swingier in both directions, but I'm alright with that. It'll be up to the players to choose when and how (and against whom) to try their skills, since they'll only have a rough idea in many cases of the level of opposition they're facing.