So this looks about like @
dnd4vr wants: proficiency is rewarded, but being "overqualified" is not worth as much more as being qualified in the first place.
This all looks right to me: something that is very difficult is very difficult even for proficient characters, still fairly difficult for specialists, and semi-reliable success is only attained by those who invest both in the ability and in expertise.
Hmm.. maybe I am missing something, but I worked some numbers this morning and this actually seems to make expertise slightly better (just a bit, but overall about a 1.4% increase from d20 to 2d10)--which is definitely not what I want, LOL! At lower and higher DCs, it won't count as much depending on the level and total modifiers, but in the middle-ground it is much better). When positive, it is an average boost of over 9%, but when negative only an average decreased about less than 6%.
Since you are using RAW, having expertise is actually still worth just as much as being proficient (they are equal, after all). Someone with expertise in an area where they lack ability score modifiers works, but since it is often used to boost skills they are already good at, to make is worse still.
One exception for Rogues/Bards I commonly think of would be getting Expertise in Investigation or Perception, particularly Perception. INT isn't likely a dump stat, but WIS isn't as often comparable to WIS-oriented classes. Athletics is like another skill for Expertise if STR is a dump stat with DEX-oriented Rogues and Bards.
I am starting to like the idea that Expertise can be used to replace proficiency or ability score modifier by matching the better of the two. So, if you have +4 prof and +2 ability, you get +4 twice; if you have +3 prof and +5 ability, you get +5 twice. If they are the same, you get both as normal, but +1.
This way a low STR or WIS rogue (or bard) can help skills with low ability, and at lower levels higher ability can be augmented by replacing proficiency. A rogue at level 1 with DEX 16 would have +6 to stealth for example because the +3 dex is better than the +2 prof.
I think someone else suggested this, but I don't have time to cite who. Sorry, but thanks, you know who you are.
Otherwise... it is back to the drawing board.
