The question isn't an easy one... because, in my opinion the most succesful developments of obscure characters ended in... them not being obscure anymore (which puts them beyond this topic's limits
For example, one of the most succesful developments of obscure Marvel character may well be... Immortus. For many years he was a mere footnote in the history of the Avengers - a one-time foe who was forgotten by every writer. But then, Steve Englehart came... and connected this ultra-obscure guy to well-known Kang. And it made Immortus into classic Avengers baddie.
Another example: Ms. Marvel. Carol Danvers, in her beginnings, was merely a supporting character in Mar-Vell's book. But then, she was made into Ms. Marvel... and turned out to be a more interesting (and enduring) character than Captain Marvel himself.
Similar thing may be happening currently in the X-books with the character of Karima. She was introduced merely as a character in the third Thunderbird's backstory... but, after few years, she was brought back - first in Excalibur
, then in X-Men
. Who knows, she may turn out a long-time member of the team.
One another example may be Henry Gyrich. I don't think that, when he was introduced in Avengers
, he was supposed to be anything more than annoying character in one particular subplot. But then other writers started using him... and Gyrich became a fixture in Marvel books. And that's good, because I really like him
But, coming back to really obscure characters... One interesting example may be Ms. Marvel foe, Destructor. He was a villain in one storyline in Carol's first book - and after that story, he disappeared. But a few years ago, he was brought back, fused to another foe of Carol's, the android Doomsday Man (which was unexpected, but interesting)... And he also turned up in current Carol's book. Given time, he may become a character with staying power...