Thessaloniki as the capital of Greece in a world where the Kingdom of Greece decides not to try an pour money into reviving a village of 4,000 into a major national capital and instead settles in the largest city in the country. If the Greek population transfer with Turkey still occurs, then Thessaloniki likely ends up with a large population of Pontic Greeks.
Frankfurt as the capital of Germany in a world where the revolution of 1848 ended in the establishment of a hereditary Empire of Germany three decades early.