A tomb raider, huh? Okay, look in the Complete Adventurer for a few PrC ideas. Actually, the Scout mentioned in there could also be a nice choice, although at this point you may be facing XP penalties if you add on another class, so perhaps that's not the best option. Anyway:
Dungeon Delver: more skill in finding and dealing with traps (including trapsense progression), Blindsense a few times per day, but no sneak attack progression
Highland Stalker: Skirmish (which works well with your Spring Attack and Sneak Attack), various movement aids (mountain stride, swift tracker, surefooted), and camouflage. Down side: no sneak attack progression or trap sense progression. It was obviously made with the Scout in mind.
Nightsong Enforcerer: sneak attack progression, bonuses to various skills, grants bonuses to some skills to allies within 30 ft, gains +20 to listen / spot vs allies w/n 60 ft, etc. This actually sounds like it might aid you most, but while it is a good add upon for a rogue - particularly a tomb raider, it does nothing for your barbarian levels. On the other hand, it does have better BAB than the Nightsong Infiltrator (which gets a few more bonuses vs stealth and traps)
Spymaster: could be fun if you use your barbarian levels to accentuate your cover identity, some sneak attack progression, ability to hide from scrying and mental probes, undetectable alignment
Thief-Acrobat: I'm sure you've already considered this one, but you may want to give it another look-over. It's close to perfect for a tomb-raider style character - especially with some Dungeon Delver and Nightsong (enforcer or infiltrator) levels.