Unfortunately most mainstream boards have one PEG / PCIe x16 slot, one M.2 slot and maybe 1-3 PCIe x1 slots (usually x1, even if they are x4/x16 mechanically).

Unfortunately most mainstream boards have one PEG / PCIe x16 slot, one M.2 slot and maybe 1-3 PCIe x1 slots (usually x1, even if they are x4/x16 mechanically). Those ancillary PCIe slots are generally multiplexed through the chipset and 1-2 generations behind the CPU's PCIe generation, so they aren't even good enough for networking or another SSD (not much faster than SATA, so what's the point). Even USB-C remains a niche feature on the latest socket 1700 (Intel) and AM4 (AMD, presumably extending to AM5), despite being standard on laptops for years. Part of this is because graphics are often not on the motherboard, so the motherboard has to include a DP-in socket to support graphics out over USB-C.