I've used a 6 wt a lot for bass, and have even tossed some very large flies, that would have been easier to cast with an 8 wt. The main difference is distance. The lighter rod will be much more limited for distance the bigger the flies are. However, if you simply scale the flies down a bit, that you have used with the 8 wt, that should still work well. (Guessing you tie your own!)
Buggers, as has been suggested certainly work well, and so do many trout flies as MoscaPescador suggested. I've always liked some of the big western trout flies, particularly some of the foam hoppers, such as Chernobyls or Triple Deckers, or some of the many large Stoneflies, both nymphs & surface flies. Some of the flies used for Steelhead work well for bass too. For example, Intruders, and IMO they work extremely well in rivers & streams for bass.
You might also consider flies such as Crazy Charlies or Gotcha's, used in saltwater for Bonefish or Redfish, and other "flats" species. (Squimps are a favorite of mine for Smallmouth bass.) They can be cast with a 6 wt, and can imitate small crayfish or even baitfish. I use these types of flies with my 6 wt for White & Yellow Perch or Crappies, and have caught bass on them as well.
There really is little difference as far as the flies, except for the sizes, between what I'll use with a 6 wt as opposed to an 8 wt. Seaducers, Lefty's Deceivers , Clouser Minnows, and Half & Half's work fine for me.