All right, then. Which two?
South Africa is pretty much a lock. Democratic, symbol of anti-racism, economy pretty good by African standards. Has some influence over other African countries and is respected abroad, as long as people don’t look too closely at things like AIDS, baby-rape, and witchcraft hysteria.
Egypt stands a good chance, since otherwise there would be no Muslim countries on the council. Internationally it is mostly well-received (think Camp David), though other African countries think it is not quite African enough. The same goes for economics as for diplomacy. It is however run by a military junta, and if it ever became a democracy we could count on it to be anti-Israel anti-U.S…
Nigeria gets mentioned a lot mainly because of its large population (which may however be exaggerated), and not because it has accomplished much of anything other than giving its banks a bad name worldwide. (The “Nigerian Scam” is that country’s # 8 economic sector.)
Oddly, no one ever proposes that Congo be made a member despite its size, population, and the fact that it is typical of much of Africa. Its international relations mainly take the form of diamond smuggling and the presence of foreign mercenary armies on its soil. Cannibalism, ebola, and the French language have yet to be eradicated there.
Kenya or Tanzania have a good international visibility due to the tourism industry. Marginally democratic, no particular international impact since the 1960’s. Too bad they’re so much alike. Oh yeah–didn’t the human race originate here or something?
Libya has become a vanguard of human civilization through the “Green Revolution” of its leader, the misunderstood genius Col. Gaddhafi, who also magnanimously abandoned his secret nuclear projects last year. What’s not to like?
Ethiopia has good food, except when they’re starving, and Bob Marley apparently has some connection with the place (like the Richard Gere of pot-smokers). The Ark of the Covenant is kept squirreled away somewhere in Axum. Enjoyed good relations with Israel (under King Solomon) and the Prophet Muhammad (back when he was losing).
In order to encourage democracy, I suppose they could give the seat to the underground opposition in Algeria.
Finally, we come to the Canary Islands, a respected African superpower if ever there was one. 
Thing is, every other continent has at least two countries (not so far named) that would be better candidates in almost every way.
Europe: Italy, Turkey, Poland, Spain
North America: Canada, Mexico
South America: Argentina, Chile
Asia: South Korea, Indonesia
Australia and New Zealand: Australia and New Zealand
So in this light, I think one African member country is more than sufficient “affirmative action” for now.