I've always taken it as...
When a character is introduced on the website with a "WARNING: CHALLENGER APPROACHING!" header, then that means that character is an unlockable in the game. Ike, Pokémon Trainer, Diddy Kong, Meta Knight, Solid Snake, Lucas, Sonic, and King Dedede would all be unlockables then. And that's a totally believable list -- which would include, of course, lots more unlockables, too.
The starting roster would then be:
Mario, Link, Kirby, Pikachu, Fox, Samus, Zelda, Bowser, Donkey Kong, Yoshi, Peach, Ice Climbers, Pit, Wario, Zero Suit Samus, and/or Luigi, Captain Falcon, Jigglypuff, Sheik, and Ness.
15 to 20 starters? 8 unlockables that we know of -- with 12, possibly, hiding in the distance.
I believe that the 8 that we know of -- the ones listed on the website -- are the ones we'll be unlocking via the Subspace Emissary mode. And the rest will come from that, Events mode, AND meeting certain conditions in Brawl multiplayer matches.