If you stick to only doing capital cities you're going to burn yourself out. Cities are great, but I think they are best appreciated juxtaposed against nature. Beaches and fresh air will recharge your travel battery, and prime you to absorb all of the amazing culture that those cities provide. Otherwise I fear all of the cities will become a blur, and the subtle aspects of what makes the city great will be lost.
Consider taking a few days to visit a place like Cinque Terre between Rome and Venice. Consider taking a break from paris to explore the vineyards in the south. Magic can occur outside of the big cities, small villages and hillside retreats can be invigorating.
However to each his own, and if if capital cities are how you want to do it then best of luck to you!