As I previously mentioned, there are a plethora of ways to ‘fix’ thieves (beyond leaving them out of the game entirely). So I’ll show case a few, some from other versions of the game, and others from Bloggers or my own ideas.
All it Costs is Time
This option was detailed by Blackrazor in his blog. In fact, it was his series of posts on Thieves that got me thinking about this at all.
The gist of his idea (go read it when you are done here) is that all thief abilities are automatically successful (within a few small constraints) as long as the PC is willing to spend the time to use it successfully. Usually this is a Turn per ability. This means that characters are using their other resources while the Thief checks everything. The longer you spend in a dungeon, the more resources you lose and the more wandering monsters encounters you might have. This means the abilities, while guaranteed, still carry risk.
This is a pretty potent option, but probably is the least fiddly and the easiest to implement. It’s such a wide divergence from the way things have always done that I am both fascinated and frightened by it. I’d be tempted to playtest this one for sure.
M Blackburn: A Writer's Journey 