At the moment entry is restricted to certain parameters that not everyone is actually going to hit. It cuts out character concepts who would be perfectly adequate as a thief character but with an unconventional build.
I want to suggest possible access for:
- Characters with high Hide/MS
- Clerics or divine classes of the classic thief-type gods
- Characters with high Sleight of Hand
It doesn’t have to be entirely closed off. There are many reasons any character might end up in a place like that, whether to spend coin, earn it, or lose it.
edit: just as an example, I played a thief Kenku who couldn’t fit Thug into feats, couldn’t fit bluff into skills and had no real reason to use it, and didn’t go deep enough rogue to get thieves cant. Still through and through a thief, worshipped Mask, lived a full life funded by ongoing crime schemes. But the doorman won’t let the character in because he’s just another slug.