Me, personally, I would take some spellcasting levels (sorcerer/wizard) seeing as how your group is pretty light there, aiming for arcane trickster.
This is often a good mix for a sneaky/tricky character, though will reduce your overall effectiveness in your main area (rogue) in favor of becoming more of a utilitarian character. If you can get decent access to magic items, you can pick up a few wands, scrolls, etc to help 'beef up' the party then strike at close range (either melee or ranged attack) at enemies while invisible. If you are like me, and really roleplay the sneaky rogue type, wizard levels are great because you can supplement or add to your sneakiness with a whole host of non-combat spells.
If you just want to be good in combat, that will take quite a while and staying rogue or picking up fighter is probably a better option. Really depends on how you play your character and what you want to be good at.
I have an arcane trickster who is just a joy to play; nobody ever knows what he is going to do next, and even with the 3.5 spell changes to haste, etc, he is still a very effective character.