Many attempts have been made over the decades to "fix" the BX Thief, widely criticized for being the only class that's actually terrible at what it does. I won't go into details here, plenty of people have done so, and there are many options out there for the thief if you desire them. I present only one option here that I came up with.
First, I want to say that I have long believed the "saving throw matrix" of BX, OSE, and Dolmenwood (and other clones) go far underutilized. This set of target numbers could be used for all sorts of things, and yet it seems no one has thought beyond using them for their original purposes.
This "thief fix" aims to use the saving throws as the target numbers for thief skills. This allows all classes to attempt thief skills, something that also is criticized widely (why can't a fighter try to hide?). The major difference is that the thief gains advantage on their skill checks.
For the uninitiated, advantage is a popular mechanic where you throw the die or dice twice and take the better of the results. Thus, a thief making a Stealth check would throw 2d20 and take the better of those rolls.
This gives the thief a large advantage over the other classes in "thief skills" but still allows the other classes to attempt them at some chance of success. Thanks to the variation in the saving throws, it also adds a neat twist that some classes are better than others at certain skills. For example, in this method, a Fighter is slightly better than others at climbing and a Cleric is slightly better at delicate tasks like picking locks. The magic-user, by contrast, is not very good at any of them.
Below is the chart. I had to make it an image thanks to this platform's formatting limitations.
