What Axler said.
For the big ones, you might want to pick an old, currently unused character and propose a relaunch.
But to echo the final comment above, don't hold your breath.
Best way is work your way up through indies and make contacts who have contacts and network. Those can be indies you create and publish or ones you work with for others.
It's a staggeringly difficult business to crack at the best of times.
Good luck.
EDIT: Oh, you should try working the cons. Go, meet people, artists and editors and other writers and work the contacts there too. Putting a face to a name goes a long way to helping the process.