I think it would be a slam dunk as a WHS, yet I haven't even heard anything about it being put on the list.
In fact Ethiopia submitted Yeha as long ago as 1978 and tried several times after that before removing it from its T List. See the history on this Web site Under "Former Tentatitve Sites" (With a review as well!) - http://www.worldheritagesite.org/sites/twhs.php?id=16
Ethiopia started its "World Heritage life" at a great pace and, by 1980, its nationalistic "Derg" government had managed to get 8 sites inscribed (Even USA which was also extremely "keen" on WHS back in those days before it left UNESCO in 1983 only managed 7 in the same time!). At which point things started to go very wrong with famine and civil war and Ethiopia had other things to concern it. The comment by the reviewer about the anti-Tigray bias of the government may have been true later, but didn't seem to prevent the original nomination - we certainly don't know why Ethiopia "gave up" on the site and removed it from the T List and further attempts. Eritrea (which was part of Ethiopia at the time of course) believes the same about the potential WHS within its boundaries - see my review of the site of Qohaito - http://www.worldheritagesite.org/sites/twhs.php?id=5600
In an ideal world one might hope that the countries of the region - Ethiopia, Eritrea and Yemen would cooperate to put forward a transnational nomination to represent the Kingdom of Saba whose remains straddle Arabia and Africa including sites such as Marib and Yeha - but, given the state of the region, it ain't going to happen! In any case the "WHS world" has moved on a long way since Ethiopia removed Yeha from its T LIst. The last Ethiopian T List revamp took place in 2012 with Yeha not getting back in. The list is quite small (5) so, with some success between now and the next revamp Yeha might make its way back?
Generally the relative lack of sites in Africa is more due to failure/inability to bring them to inscription than a lack of T List candidates. Ethiopia's orginal reasonably successful inscription "rush" reflects to some degree the lack of very careful scrutiny of Management Plans etc in those early days of the scheme.
But, whilst agreeing that Africa does contain a fair number of good potential WHS I am rather wary of arguments which rely too much on factors such as the need, because of fairness and avoidance of cultural discrimination, to achieve "equity of inscription density" across regions and countries. Each continent, region and country has its own specialisms, and the World Heritage concept is built primarily around tangible Cultural heritage with Natural heritage slightly uncomfortably "tagged on". IUCN has been rather stricter than ICOMOS in trying to avoid too many "duplications" of the same sort of geology and ecosystem and this partly explains why many fine National Parks in Africa are missing. It needs to be remembered that, whilst Cultural sites ONLY have UNESCO WH as the international standard for recognition and preservation, Natural sites have a range of other schemes - UNESCO Geoparks and Biosphere Reserves etc. And African culture is likely to be "disproportionately" represented on the Intangible Heritage list. UK isn't "complaining" that African countries are better represented on that list than it is!!
The answer isn't to keep identifying more and more areas of tribal "Cultural landscapes", rock paintings, sacred forests and wildlife parks in order to achieve parity by land area and population for Africa with other regions - but i fear that this is what is going to happen!