Who knows?
I HIGHLY doubt they will add additional gamemodes. It's just not SD's style.
SD does class-based, team objective games, period. There's also these quotes from devs to help solidify my argument:
"We know the game will be enjoyed by Enemy Territory fans, people who play team class objective based shooters. We’ll know they’ll like it. We’re trying to pull in the more main stream people. That’s why we have things like iron sights and our controls are very similar to those major shooters. Then we’ll introduce more advanced elements like “No this isn’t just capture the flag. We’ve been doing those modes for 20 years. There’s something more interesting why don’t you give this a try."
source - http://gamingphanatic.com/2011/01/07/brink-splash-damage-interview-with-neil-alphonso/
"There are CTF-like mechanics in some places of Brink, but we won't ever do 'pure' CTF because it'd be like taking a step backwards from what we're trying to do."
source - http://www.splashdamage.com/forums/showpost.php?p=222132&postcount=15
One of the appealing things about Brink, is that it is not marketed to "slay the giant." - It is not attempting to take on COD, or outsell it (like BC2 blatantly was)
Brink is its own game, with its own priorities and own fanbase. They are not trying to take over the market, and that's ok. There are perfectly content with not being number one, and this approach allows the developers to make a game they want to play, with their own ideas and visions, without compensating for the sake of sales.