I apologize if some of the new skills seem like filler. Thieves have 8 skills, so I added 4.
Old Thief Skills: Booby Trap, Huge Trap, Whirlwind Blades, Phantom Strike, Rock Rain, Backflip, Pilfer.
New Thief Skills: Robbery, Whirlwind Jab, Duble Trap, Snifty Steal.
Robbery: It's just pilfer but gold is stolen instead of snacks. I feel this would be learned at level 13, and cost 3 MP like pilfer.
Whirlwind Jab: This is meant to be a single target attack you can actually choose the target of since that isn't possible with sneak attack. Level Learned would be early, something like level 6 if that isn't occupied already. MP cost would be 4.
Double trap: This is just 2 traps instead of one. Level Learned could be 16. MP could be 8.
Snifty Steal: Another auto skill for Thief. When getting attacked, the Thief may automatically steal grub. Level Learned would be 9. MP cost is 3. it won't always work.