If you are planning for two months than I think that you can add Spain. There are a lot of budgetflights available to there. You could take a flight from Paris (or maybe directly from London to Madrid or Barcelona), and than add Paris after Amsterdam. With a open jaw ticket you could then return home instead of returning to London, which usualy is a hassle as the intercontinental flights arrive and depart from Heathrow, and the budgetflights go to one of the other smaller airport. (Eurostar would not be so bad, as you can go from the station to the Airport from there, but it still is quite a distance.) If you really want to save on backtracking see if your airline makes it possible to return from Athens, as that is the most "remote" destination.
Also check how you want to get to Greece. From Italy there are a couple of flights that are cheap. I think the cheapest ones are the Easyjet flights from Milan. But also from Naples and Rome budget flights are possible to and from Athens. However if you go to Greece, at least take the time to visit two of the Islands in addition to Athens. (As you wrote Greece I suspect that you will chose for more destinations.)
If you want to add Spain, a classic is to go to Madrid - (Cordoba) - Sevilla - Granada - (national flight to) Barcelona, and from there it's easy to find a flight to Venice, Florence, Pisa of Rome. There is also the possibility to add Portugal into the mix (e.g Lisbon and Lagos.)
If you skip Spain you can also make so additions in the countries you are already visiting. like Cinque Terre in Italy. (depending on where you go in the Alps, it can make a good connection), also in Italy it would be an idea to add a place on the Amalfi coast, and visit Capri and Pompeii from there. I presume that in your initerary the connection between Venice and Berlin is a flight, you could also add: Innsbruck, Salzburg, (Munich), Prague, Dresden and go by train. All of course depend on the time you have, and want to spend in each place, and if you think you must visit them at all.
happy planning.