I currently use expansion for soundcard, and a video capture card
what I'm interested in doing soon is adding a NIC for LAN teaming (some motherboards already include two) to and from a NAS, although 10GbE would be incredibly awesome, I just don't see that coming down to consumer level pricing for a few more years
NVMe PCI-e SSD will be amazing as well, and I could see myself being an early adopter for that.
a hardware RAID card was something I wanted in the past, but SSDs and improvements in onboard controllers have kept me from making that plunge.
Intel has dropped the ball a bit with Thunderbolt, but there's always a possibility things change there and it becomes desirable, and a PCI-e card with Thunderbolt (assuming compatible motherboard) could be something I would be interested in.  Thunderbolt 1 = 10gbps, 2 = 20gbps, amazing for DAS solutions or even displays or as an external interface with PCI-e devices
And in similar vein, there's also USB3.1 on the horizon, which very well might find its way onto some of the motherboards you might be considering, otherwise I imagine there will eventually be PCI-e options.  3.1 should double the speeds of 3.0, so 10gbps up from 5.