I'd love to see them follow through the Celtic, or ancient European ideas a bit further - imagine a Wicker Man themed episode? Maybe a little too adult for Sg1, but I am sure it could be adapted.
Also, I always thought the Greek gods would have been perfect fodder for the writers' imaginations for Goa'uld templates. They were always bickering and intriguing against each other. I know we have seen a few instances of Greek-themed gods, but how cool would it be for them to really go for it? Imagine how they could explain the birth of Athena (she emerged fully grown from Zeus' forehead) as related to Goa'uld reality?