Idea for next multi city trip

Hi. In the summers I get around 2 full weeks off work, and like to do a multi-city type holiday (I'm from UK and travel alone).

I've enjoyed doing some multi city travelling over the last few years.

For example Tallinn-Riga-Vilnius in one trip and Copenhagen-Stockholm-Oslo in another. Also Budapest/Vienna/Ljubljana/Venice more recently.

Cities/Countries I've already 'done': Oslo. Copenhagen. Stockholm. Iceland. New York. Munich, Barcelona, Madrid, France, Venice, Ljubljana, Trieste, Vienna, Budapest, Prague, Sofia, Istanbul, Tallinn, Riga, Vilnius, Bratislava, Krakow, Switzerland, Jersey. Berlin. Hamburg.

I'd like to start planning my next one. I've done most of the 'obvious' countries though now and don't like going back when there is so much to see of the world.

More of a fan of Europe, as it's easier to cover the distances.

I enjoy the typical city breaks. I don't want to rent a car as don't want the stress from that. I prefer to travel by train or plane and sometimes bus if I must.

Love to hear some thoughts/ideas? I tend to stay in a place anywhere between 2 and 5 days depending on how big the city is before moving to the next.

Thanks!