You can visit Florence between Rome and Venice.

Stop in Dresden between Venice and Berlin. Potsdam is an easy day trip from Berlin (about an hour away).

I don't think you'll have time for off the beaten path destinations. Normally one needs a car to access these places and they're not usually for first-time visitors to Europe.

If you really want off the beaten path you can do this within cities - go to areas that have no tourist attractions and wander around.

Paris -> Rome -> Venice -> Berlin -> Amsterdam -> Bruges/Brussels

Venice to Berlin is a long long haul - you could take an overnight train Rome to Munich then a several hour day train to Berlin. So yes stop in Munich for the day - take the train out to Dachau to see the camp (overnight rule on trains means that you railpass would be valid the whole day you arrive in Munich - you could also use it to go stay the night in Rothenbrug if not into Munich then go onto Berlin from there, etc.