I can't quite remember the thread here, but I do remember someone on this site mentioned that Ethiopia's geography not only helped shield it from conquest (especially from the Muslims right across the Red Sea), but also discouraged expansion in most directions, asides from along the Red Sea coast towards the east. If even a semi-powerful state was sitting there, knowing the Ethiopians could only come from that direction, it would be easy to keep them from expanding there.
It would also help if they could maintain a cozy trading relationship and alliance with the Eastern Roman Empire, while also being a bastion of Miaphysite Christianity far from the reach of Constantinople. The ERE, even despite this, would probably do more business through the Red Sea to get to India instead of constantly going heads up with Sassanid Persia. If they could spread their version of Christianity up both sides of the Red Sea, it could possibly butterfly the rise of Islam. If they could somehow manage to expand down the Nile to the African Great Lakes and simultaneously towards the Swahili Coast, they could really cement themselves for a very long time and encourage further subjects to further assimilate.