Although I'm not much of a thief player myself ill try to give potentially helpful ideas,
To me the process of coming up with interesting mechanics starts with defining a clear identity/concept for the class.
Thief in a D&D/RPG context is vague.
It sometimes is a actual burglar, sometimes a type of "swashbuckler" (agile fighter), sometimes a kind of scout.
In this though exercise I will define a concept I personally find more interesting and go in depth with that one in specific,
but the same process could be done for any interpretation of Thief. They could even be used to create sub-classes for them.
I will be going with a swashbuckler, which is a combat oriented interpretation of the concept (I never liked the entire stealing
tems mechanic, and scout has some overlap with rangers).
--------------Swashbuckler thief concept -----------------------------------------
In our imagination a swachbuckler is a agile, lightly armored warrior which duels with a light weapon like a rapier
using footwork, feint, and sword skill rather than armor, brute force.
To express this, i would add a mechanic which improves the swashbuckler's fighting ability based of his load (as in weight),
the lighter the load, the bigger the bonus.
This will incentivize players to pick armor with low weight and really embody the concept. I'm not sure what is balanced so
I will give a arbitrary example here to illustrate:
say this "swashbuckling" skill gives a maximum bonus (aka naked theif) of
-300 armor
+20 hit + 20 dex + 20 reflex save
And some nice chance to proc a extra "swashbuclker's flourish" attack (say 30%).
As nice bonuses as these are, it will never be worth not wearing anything.
And for every (say 20lb) of weight the player carries, he takes a say 2% reduction on these values (armor, hit, dam, chance to proc extra attack, etc).
So in effect, these thieves will wear armor but weigh in the bonus from the pieces with what they lose from their swashbuckling to try to retain as much as possible.
In the end they might realistically end up with a much more modest bonus of something in the -100ac, +8 hit/dmg/reflex, for example.
For this type of thief it would make sense to have dodge, parry, dual wield, feint. But not shield block. Weapons i think swords, daggers, hand to hand and whip proficiencies make sense, but would skip the heavy ones like maces and axes.
I see them having both sword and dagger skills (swashbucklers typically would have a rapier and a main-guache, so it would be a sword + dagger combo), and perhaps a few
custom skills for this specific weapon combination. Perhaps a blinding strike done with the tip of the blade, improved disarming ability, perhaps some sort of "armor piercing lunge" which ignores/reduces targets resistance.
I could see them even having a species of "craft" skill similar to "craftharpoon" from spear spec fighters to make rapiers out of swords they find and have these skills work only with these rapiers. It could make the swords much lighter (perhaps a fixed 3lb or something), but not count as swords for other sword skills (so other classes would not want them).
Once you have this sort of theme, it is easy to think of fun and flavorful skills which really give a class a "soul", even if mechanically irrelevant. I could totally see these swashbucklers have a "swashbucklers mark" ability where they brand a enemy with their initial (Z for ZORRO style), which sure could do some damage, but the fun part would be to give the target some sort of "tattoo" (visible by looking) which has your initial,
Another possible very unique mechanic which fits the concept is losing resistances. In my imagination swashbucklers are agile and skilled but total featherweights - can't really take a hit. So the swashbuckler class might reflect this by losing all resistances and gaining a AC bonus based off the resistances lost (or maybe bonus to dodge/parry), The idea would be to make them very elusive, but when hit get hurt bad.
Perhaps some sort of "active" feint skill; skill which lags you (say 2 turns), but deflects the next active skill/attack made against you (so there could be some strategy to it in the sense that it will be strong against something with higher lag time, but bad against something with lower). Or if that is too powerful, it could dampen the damage from the next active skill when used. Thinking something like this would probably be important to compensate the resistance loss above.
I'm just throwing ideas which fit this swashbuckler concept in particular, if there is interest we can brainstorm more and try to refine the specifics in a way which is balanced (im just making up the numbers off the top of my head). But to prevent this post from being gigantic, ill stop here as i think i illustrated the concept and process i would take in creating a class.




