Paris to Berlin

Just about any place you care to visit in Belgium, Maastricht (Netherlands), or Cologne (Germany) would be on or close to the direct train line. And I can't resist mentioning Quedlinburg (Germany), which would require a considerable detour but is a UNESCO site. It has a sizable, to-die-for historic center full of colorful half-timbered buildings, a castle, a small art gallery and a church treasure with an American connection. The tourist office rents English-language audio guides, and I think there are also English-language tours. Others can probably recommend picturesque towns considerably closer to your direct route, but Quedlinburg has more medieval buildings than most of the alternatives and is almost unbelievably pretty.

Paris to Luxembourg for Trier (1).
Down the Mosel to Koblenz (2).
Spend time exploring the towns along the Rhine
Head to Cologne (2). Side excursion to Aachen.
Lastly Hamburg (2) before Berlin.

Certainly depends on your own particular interests. With a week to get between Paris and Berlin the possibilities are endless. If there is weekend within this period, I would head to the Mosel Valley to a location with a Strassenweinfest going on. It is the grape harvest season, and these are particularly fun.
Add in a day cruise on the Mittelrhein, and any number of old villages like Quedlinburg. Nuremberg, Erfurt, Leipzig, Dresden, even Prague are possibilities.
I'd steer clear of night trains. There really are not too many left, and distances are short.

You're seeing two great cities! I have two ideas for things to do in between Paris and Berlin.

If you're interested in World War I history, a good idea is to tour a WWI battlefield. I have always wanted to tour a battlefield in part because my grandfather served with the US Army during WWI. Google WWI Battlefield tours, and a number of tours come up in Eastern France. I was in Belgium once and wanted to tour the Ypres battlefield site, but lacked the time. This was the tour listed in my guidebook: https://www.ypres-fbt.com/ . There is a museum in Ypres, too.

You are seeing two great cities in Paris and Berlin. When I travel, I try to spend a couple days in small towns. In Germany, there are a bunch of great small towns. Rothenburg ob der Tauber (http://www.tourismus.rothenburg.de/?id=467) is like stepping back a 1,000 years into the Middle Ages, but it's crowded with tourists, though still worth seeing. I always tell people to go to Unesco-listed Bamberg, which is in the northern part of Bavaria (http://en.bamberg.info/). It feels almost as medieval as Rothenburg, but has a third of the crowds. Bamberg has some great history and culture, including a superb beer-brewing museum, the ETA Hoffman Haus, a great church, a great palace and associations with Claus von Stauffenberg. ETA Hoffman, who lived in Bamberg for four years, was a great German writer in the early 1800s whose works include the original Nutcracker story. The church has a Tilman Riemenscheider altar. He is sometimes called the German Michelangelo. Riemenscheider carved stunning wood altars. The church also has the only pope interred north of the Alps. Finally, Stauffenberg was the German officer who came very close to assassinating Adolf Hitler in a bombing, but I do not know if Bamberg has any tourist sites involving Stauffenberg. His family lived there.

The Stauffenberg family chateau (Schloß) is located between Albstadt and Hechingen, which has a museum on the resistance.

@ alexishkeller....If you want to do part of the ride to Berlin on a night train to squeeze out another day, there is indeed that option, depending on where you start that ride. On the other side of Strasbourg, take the night train CNL Offenburg to Berlin. If you decide to go Munich to spend some of that free time, there's also Munich to Hannover Hbf, then change to Berlin.

I'd like to suggest a stop in Luxembourg. Most people just pass by it because it isn't in any RS guide books. However, it is a very interesting place and deserves a stop. It also shouldn't be very crowded.

Overnight trains to Berlin don't depart from France nor from Munich, but are available from Zurich, Basel, Mannheim, Cologne, or Vienna, for instance.

@ Laura....Are you saying that Munich to Hannover Hbf night train doesn't exist anymmore? I took that in June, then transferred in Hannover Hbf to Berlin. True, the Munich-Berlin CNL train went out in Dec 2015. I took that one (very nice, ie nicer than the Munich-Hannover) at the end May 2015. There is still the night service on the IC or ICE , just minus the "sleeper and couchette" accommodations ( I never used them anyway), right? One can still go to Berlin by night, take Munich to Hamburg Hbf, transfer to Berlin on the ICE

Or you could go to Strasbourg/Colmar, then Frankfurt (Mainz/Wiesbaden) or Munich, then Berlin.

A few nights in Wurzburg would allow day trips (by direct trains) to Rothenburg ob der Tauber and Bamberg.
A few nights in Nuremburg would allow day trips (by direct trains) to Bayreuth and Regensberg.