Ah, I apologize for misinterpreting your statement - if you want a Dynasty interested in urbanization, you could have a much more stable and powerful Zagwe Dynasty emerge but I'm not completely sure how that can be achieved, although my recommendation about the Zagwes and Solomonids entering into personal union could work.I never meant to give the impression that Khartoum would be a capital. Just the northernmost city. But from the sounds of it, the issue was basically deciding to. So we need a dynasty willing/interested in urbanisation.
In that case a better PoD could be to consider changing behaviour in Aksum, have them either respond early, or invest in advance of 650 AD - establishing lands west of the Highlands could provide a more sustainable and less fragile agricultural base. It butterflies a lot of history, but even if it doesn't save Aksum, a local Ethiopian dynasty could emerge.
You could have Axum successfully conquer and annex Christian Nubia while focusing on its integration into Axum proper - the rise of Islam and Queen Gudit's destruction of Axum might see the Axumite Royal Family and government relocate to Nubia where they focus on rebuilding their forces before launching an invasion to retake Axum - again, I have no idea how plausible this is.