While it is technically possible to design a card that would plug-in to the PCIe bus, it would also have to provide graphics capability or have an input to provide graphics from another source. The Thunderbolt connector has to provide power + PCIe I/O, and DisplayPort graphics.
ExpressCard cannot meet these requirements.
Still not able to provide the 10watts of power from the ExpressCard slot; would require an external power supply.
And as both you and I state in different ways, will also not meet the performance requirements.
I assume the original question was about the ExpressCard slot on older, existing laptops. Not some new enhancement of an ExpressCard implementation.
Bottom line, it's not going to happen - there won't be any Thunderbolt card for current ExpressCard slots.
Thunderbolt is the new expansion capability for laptops, and some desktops and all-in-one computers.