- Dwarves have a 2-in-6 chance to find slanting passages, traps, shifting walls, and new construction.
- Elves have a 2-in-6 chance to find secret doors.
- Halflings have a 2-in-6 chance to hide in shadows. (Let's ignore the wilderness version.)
That's awesome stuff of course. The only problem is that it stays that way forever. You can scan the B/X Expert Rulebook all you want, those special abilities don't improve! Doesn't matter that your dwarf is level 12, your elf is level 10, and your halfling is level 8: 2-in-6, 2-in-6, and (cynical drumroll!) 2-in-6. Depressing. Possibly one of the worst oversights in all of B/X?
Luckily it's easy to fix: Just tie those chances to the demi-human's level and you're done! There's already one (and only one!) 2-in-6 ability in B/X that improves with level: the "hear noise" ability for thieves. It goes to 3-in-6 at level 3, 4-in-6 at level 7, and 5-in-6 at level 11. So just declare that demi-human abilities also improve at those levels and you're done! You now have a better, more demi-human-friendly B/X to please your players with. Yay!
Of course using "hear noise" has the strange consequence that elves and halflings don't get to "max out" their special ability. I can live with that, but just in case you can't let's try to find another way to read B/X that might make you happier. One idea could be to look at XP instead of levels. Thieves reach level 3 at 2,400 XP, level 7 at 40,000 XP, and level 11 at 400,000 XP respectively. What do those numbers mean in terms of demi-human levels?
Alright, so halflings still get the shaft and never reach 5-in-6 for hiding. Worse than that, elves now start with 3-in-6 for finding secret doors. I don't actually like this better than the first approach, but hey, maybe you do?
In my house rules levels 4, 8, and 12 are "special" because they coincide with the level limits and level progressions used. So there I reuse those numbers to determine when demi-human special abilities get upgraded.
Of course the important point for me is not which progression to use, it's much more about adding another small way in which characters improve over time. Take it from someone playing a level 12 dwarven fighter in AD&D every week: Nothing is more boring than (apparent) lack of progress!
PS: I don't think that any of the other "general" d6 skills should improve with level. But for demi-human special abilities, I find the idea rather appropriate (and indeed appealing).