There are likely others than what I'll mention here that would be interesting to me, but these are off the top of my head.
1. Xander & Willow
Maybe this is a hot take, but I always thought this was a couple I would have liked to have seen explored in season three. I love the arc the way it went, but I have a strong curiosity as to what shape that might have taken. I think, as best friends who love each other more than anyone (nobody messes with my Willow), they were well suited for each other, complimentary to each other's personalities, strengths and weaknesses. That said, I don't think this should have been a permanent relationship. It would have been a nice one, though.
2. Gunn & Anne
I agree with you here, Britt and Joanie. They had chemistry, they understood each other, and they both have history that gives them a unique perspective on the youth runaways topic.
3. Kate & Angel
There were definitely feelings between these two, and it would have been nice if, in season four, they had found their way to being friends, and then perhaps something more. Obviously, it wouldn't have been a lifelong (or series long) relationship, but it could have been nice for awhile. Unfortunately, Elisabeth Rohm got cast on Law & Order, and whatever the plans were for Kate, they did not come to fruition as a result.
4. Cordelia & Angel
Post Doyle's death, as Cordelia began to mature, there was definitely potential here, and the show obviously toyed with the idea, as we can see. It would have been an interesting relationship to witness.
5. Fred & Wes
Though we did see them get together, we never saw the relationship in a real way. I know not everyone sees this as a legitimate relationship, but I think they had great chemistry, a lot in common, and they obviously loved each other.