Intel's next-gen Alder Lake-S desktop CPUs will require a new LGA 1700 socket according to a leak from the company's official development resource website which lists the various projects that the firm is working on. Good ol' leakster momomo_us was able to grab a snap of the information there about the future Alder Lake platform(s).
Intel only recently debuted LGA 1200 socket and Z490 chipset motherboards with Comet Lake-S and so it'll likely be a while before we see Alder Lake-S and socket LGA 1700 in action.
Rocket Lake-S, the immediate successor to Comet Lake-S, could be the first Intel platform to debut PCIe 4.0 support according to a leaked features slide (via Videocardz). Hence, the recent inclusion of PCIe 4.0 support on Gigabyte Z490 motherboards makes sense as it could be in anticipation of Rocket Lake-S and sort of confirms the existence of PCIe 4.0 compatibility on the next-gen Intel desktop lineup. So, Alder Lake-S might be when PCIe 4.0 support matures and expands to more affordable motherboard options.
There is also the mention of Alder Lake P but currently, no concrete information exists about it.