[mikepurvis]

A thunderbolt 3 connector is 4 PCIe lanes, isn't it? I know there can be compatibility gaps, but there are definitely TB-connected enclosure boxes available. NVMe connectors are also 4 PCIe lanes, and I believe any of those can be broken out and used for whatever (m.2 cellular data modems for example).

Are you thinking of plugging in actual consumer expansion cards, or are you wanting the lanes broken out on some kind of riser where they can go to hardwired stuff on a carrier board?

[wtallis]

Thunderbolt isn't literally four PCIe lanes; Thunderbolt can encapsulate and carry PCIe traffic, and Thunderbolt controllers are typically connected with four PCIe lanes, though the amount of PCIe traffic a Thunderbolt link can carry is not necessarily as much as four PCIe lanes.

Directly exposing literal PCIe signals cuts out the pair of expensive Thunderbolt controllers.