Drehscheibe - alles zur Eisenbahn, zu Zügen etc

Imo this is really very very unlikely to happen unless they completely rebuild Köln Hbf or send the Eurostar to another Köln station and rebuild that one entirely or close it off to pretty much all other trains. You need room to accommodate hundreds of people for multiple hours, like in an airport terminal, in=in, so you need separate toilets, shops & restaurants and so on, +room to accommodate controlling personnel and their equipment too. It would cost many tens of millions if not more then a hundred million euros.

Köln platforms are barely large enough to accommodate people for regular ICE without border control stuff. Even after big investments it would probably mean more half full trains, just like on the London -Amsterdam, the passenger limit being the terminal size, not the trains.