Last I heard, Blackbeard was looking to defeat a really powerful pirate to be made a Shuchibuki. Then fought Ace.

The anime hasn't said how that played out. But if Blackbeard defeated Ace, I'm guessing he'd be made a shichubuki.
Quote Originally Posted by Kraco
Enel was probably still far from his actual potential because he had rotted so long up there in the high skies with nobody tough to fight against. If he had been living in an environment with numerous tough opponents, like the greater pirate captains and admirals have, he would probably have been a lot harder opponent. If you have little experience and willpower it doesn't matter how powerful you are theoretically when facing badass opponents.
Or he'd have been beaten by somebody strong alot sooner. Growing stronger through fighting tough opponents requires you to not be dead or imprisoned after doing it.

The Strawhats keep getting stronger because they keep winning. If they ever lost...well, series over.