Cologne Frankfurt would actually be a cool route, the only problem is that there isn't alot to do so I would prefer the join up for cologne-duisburg
The reason why I voted for "Frankfurt-Cologne" is to extend the route. The speed of Frankfurt to Cologne ICE is said to be 300. Of course, I hope that both routes are in the DTG route map.
Rheinstrecke!!! Frankfurt to Koblenz - both sides of the river, it does not get much better than that.
I would love to have either Hamburg to Bremen, Hamburg to Berlin (via the old line), Hamburg to Cuxhaven, or Hamburg to Hannover. Simply put, anything with Hamburg docks and Maschen Yard! although, I would like to have a route like Kiel to Padborg (DK) which would then introduce International services