I want to test one
My TL: 700 AD the Tibetan Empire invades China then collapses early, Tang Chinese have early discovery of gunpowder and extra expansionism and awesomeness, absorbing Tibet as vassal and reaching Caspian coastline and downbeating both the Sassanid Persians and Arabs. Greater east-west exchange occurs.
Byzantines quickly adopt gunpowder and firmly reconquer the whole mediterranean area by 900 AD in alliance with super slavic kindgoms of upper balkans and lower ukraine. Frankish Empire is confined and focused on central france and germany, fights off vikings who turn to Britain as a softer target, and look more towards exploring the atlantic.
1100 AD, chinese awesomeness decreases back to normal, advanced slavic kingdoms seek Byzantine aid in wars against steppe incursions, allowing Arabs to invade Egypt again, economic turmoil. Frankish empire becomes heavily armed and expanded HRE style mess from atlantic coast to poland. Viking colonists get a firm toe-hold in canada.
1200 AD, Vikings of scandinavia (and bits of britain) united in one great kingdom and christianised, burgeoning trade coming from north america. Byzantine holdings in spain, southern france and northern italy are breaking away again, exploring into the Atlantic to seek their own trade routes...