Whatever you do you will have a fantastic trip! It all depends how much time you have! I did the same trip you did but starting from London and taking the train all the way to Greece and what a trip that was.
I have lived in London and now live in Paris and I would say these two cities are both absolutely "musts".
For a bit of ancient feel, Rome is great and all the Italian cities mentioned are beautiful. Venice is quite smelly in summer due to drainage issues, but it is romantic.
I also love the south coast of Spain. If you want a bit of glitz Puerto Banus on the Costa del Sol is a lovely place to stop. You can even take a day trip to Tangier in Morocco from Costa del Sol (Tarifa). You will need a car.
If you like French wine : Burgundy area (near Lyon), Bordeaux or chateau neuf de Pape are all great places to sample the best wines.
I love champagne and a trip to Rheims or Eperney (2 hours drive from Paris) is unforgettable. Have a great time wherever you decide to go!
Ps Budapest is also a fascinating place if you are willing to including Hungary on your tour!