I know that many folks are commenting on the transportation aspect of your trip -- so I'll focus there, assuming that you're looking for suggestions on cities to visit vs. things to do in those cities. I'm also guessing that you're either using FF miles or got a great airfare, so perhaps were restricted on that front to the selected citiies.
If you're set on those cities (Munich, Venice, Rome, Paris, and London), then the goal becomes a matter of making the transportation between the cities as easy and smooth as possible - maybe even paying more to do it. Night trains? Airlines (you'll lose a day)? Experience trains to get you point A-to-point B?
If you're willing to remove cities off of the list, then you've received a variety of suggestions. I've spent a week in London and 4 days in Rome (different trips...and other cities as well). I haven't been to Munich, Venice or Paris before -- and I have made decisions to drop desired cities off of a list as I decided that it was too much logistically (which I think others are pointing out).
Regardless, I think that I would keep an interesting city in or near Frankfurt/towards whereever you go next as Night 1 of your trip -- in my opinion, I always sort of expect something to go wrong with travel and need flex on Night 1. You need about 24 hours flexibility time for things to go wrong (in my opinion). I don't know enough about Frankfurt/Munich/etc. to advise on that. I'd probably look for something with good transportation connections to your next city.
If you're wiling to consider different cities - consider keeping it in Northern Europe for ease of travel -- a few days in Germany, and 3 or 4 days in/around each of London and Paris. You might also decide that London and Paris are gateway cities that you'd more easily get back - and elect to do a day in Germany and go immediately to Venice, Rome, and spend a final few days in London.
Bottom line, I think that you should either focus on London and Paris OR Italy and London. It's a tough call -- I love Italy -- but I think that I'd probably do the Northern Europe thing.