So apparently it is now “official” that there will be an 
Antwerpen–Köln service starting in September this year. On VagonWeb, they can already be spotted with the train numbers 310/311/312/313.
What I did *not* know is that during the summer period (01/06–31/08 — before the Antwerpen service starts in September), there will be a daily between Oostende and Köln. The train numbers are 1110 and 1111.