A good Harold Godwinson survives TL, I mean there is one but it updates so infrequently and it has a very limited scope (still very good but very focused), I'd love to see a long term Anglo-Saxon TL.
A powerful HRE that unifies and centralizes during the middle ages similar to OTL France.
A TL where by coincidence both the Emperors of Rome (Constantine) and Persia () convert to Manichaeism or a similar faith. Them both converting at the same time is seen as a sign from God to work together. While no true union is formed there is less direct conflict, and so when pagan barbarian horse people from the north start affecting both of them, we get to see the Roman Legionary and Persian Immortals side by side Crusading against the Steppes. It could be a slightly realistic start to creating a TL with only One World Religion, mighty and glorious Manichaeism.
I don't know about Persians but Byzantines never would accept infidel emperor.
Über Switzerland with a post-1900 PoD
Something with San Marino
A timeline where Indian kingdoms absolutely dominate the Indian Ocean and eventually colonize Australia. This forum needs more Indian TLs.