b) Create some kind of reputation mechanic where even successful thieving eventually gains you a reputation as a notorious thief. So even if you don't get caught, there would still be some consequences for getting all that stuff for free. Merchants everywhere would stop trading with you in a normal fashion or offer bad prices. If you want to steal stuff without getting a negative reputation, wait for night time, sneak into stores and make sure there is no one anywhere.
That would be an opportunity to include "Fences" (People who buy stolen goods) in a Thieves Guild f.e. Mol's new Thieves Guild in Baldurs Gate City or other Areas where you can Trade with shady people, even Zhentarim or the Trader Goblin in Goblin Camp, who talks about a stolen Ring fomr the Waukeens Rest raid.
So much opportunity to make Thieving more interessting. Its just needs this balance of having a Thief Party doing this and having another reputation as a notorious Thief instead of good standing with Standard Merchants.
I could even think of making the Thief himself allways disguise with the Spell, so to have him not caught by guards or be remembered, becuase nobody knows his original Face (like "No One", from A song of ice and Fire Books who was an Assassin)