I'm just about to pull the trigger on a second 700c wheelset. What I did was ask my LBS if they had any "takeoff" wheelsets identical to the one that came on my bike, and they did!
So I'll have two wheelsets with identical hubs. I'm going with an 11-26 (maybe 11-28) SRAM cassette. The cassette isn't important, just the position of the brake rotors relative to the hub.
I'll be using 40mm tires that are faster then my GK SKs. Although I'm tempted to go with say a 32mm GP5k, I will be doing solo endurance and not group rides.
I thought about going 650b/700c but for this year I'll be doing more long rides on pavement. My "gravel wheelset" will get use on my daily gravel out-and-backs and up to 100 mile rides. I've done one gravel century so far, and I thought it might be nice to go 650b. I have some friends that do bike packing on gravel, maybe next year I'll add the 650b wheelset for multiday outings.