I think a german ride could still work out, however not as a tame ride like El Rio but more like maelstrom or even a downsized german Splash Mountain. Mountains and fairy tales/legends have a high value in German culture and folklore. Why not combine them in an adventurous and wild river ride? It could include several minor and one bigger plunge (although not as big as Splash Mountain) First it takes down a minor plunge into a German wood (Black Forest perhaps) where you see scenes from German fairy tales and the struggle of Siegfried and the dragon, this could be a first highlight with an AA firebreathing dragon. Then through and up a mountain (Blocksberg), watching the dwarves in their mines, pass a sleeping King Barbarossa and out again, onto the summit of the mountain and pass by a witch sabbath on Walpurgisnacht and see valkyries in the sky to the valkyries ride of Wagner. Then follows a big plunge down the mountain into the Rhine. You pass several castles standing high above on the surrounding mountains (and seem to see even some ghosts in the castles?) and see Siegfried on his Rheinfahrt again with the famous music from Wagners opera and pass the Lorelei. At the end it goes again down a plunge (Rheinfälle). Finally the ride becomes more tame and you pass through a collage of modern Germany, including famous German buildings like Cologne Cathedral, see an ICE-train passing on high speed and finally you are on the Spree River and passing the Reichstag in Berlin before the ride comes to an end.
Just my idea.