on thief skills for non thief (and in general skills)
I would so much prefer an expansion on the ability roll rule - and I should take a moment to praise it as it transform ability check in the same rule system of Saving Throw a thing that I praise and love
as for improving the probability of success with level I think it's a very good thing, (only we must remember the ability bonus) obviously we must look at numbers a non-thief will never be better of a thief even if the thief have a few level short of the character.
As one can see from the table below it's a tricky problem since the thief progression is very particular but with skill penalty I think it can be done (after all this is all in the GM section so). I would go for the following ruling:
"Non thief character can try to emulate some of the thief skill with great risk, since they are not trained in this skill they must make and ability roll (using the right ability as shown in the table) with a penalty to the roll (the ability are taken from the thief options supplement r2:
Thief skill............Related..........Non-Thief
........................Ability...........penalty
........................................to Ability roll
.......................................................
Open Locks.............Dex...............-5
Remove Traps...........Int................-5
Pick Pockets............Dex...............-5
Move Silently...........Dex...............-2
Climb Walls.............Str................-2
Hide.................... Int................-5
Listen...................Wis................-2
(and maybe also the race table for dwarf, elf and halfling?)
As for condition the mod should be the same as for thief characters (converted in d20 mods obviously).
as reference in attach you will find a table that compare Ability rolls rule page 141-142 of r77 vs Thief skills by level, I converted everything in % (17 or more on d20 is 20%, than ability rolls get +5% each 2 levels), we must not forget that on ability rolls one get from -3 to +3 on the roll based on his ability!
after the table I must say that maybe the Ability Rolls table should slow down at higher levels