Instead of adding levels or removing the cap, which would require them to rebalance the skills/special/perks values, it would make sense to drastically increase the number of xp required to get to the next level as you level up, effectively weakening the player from the start.
It would make getting to level 30 actually feel like an accomplishment if it took something like 100 times more xp to go from 29-30 than it did from 1-2 and it would probably minimize the number of people hitting the cap before they're even close to done with the main quest.
I haven't even touched the strip yet and I'm at level 22 already, which worries me..
For DLC I could see a rebalance like this, accompanied with some new items and locations around the existing map and added questlines that would all take place before the ending of the vanilla game.
This way wouldn't require them to back out of their 'definitive ending' statements, and they'd still be able to add content with ease, and make sure that the content is useful to the player and not just more filler.
As it is, it seems like they were trying to make this game less of a walk in the park than FO3, so it would be in line with what they've done so far to ratchet up the difficulty further by nerfing the player a bit from the start and adding in bigger harder enemies with huge DT in the late game like they did in the early parts of the game with radscorps and cazadors.
It would take more killing/questing to reach the higher levels and prepare yourself along the way in the main quest, and they could add areas with horribly overpowering enemies that are off of the main quest path so when you're nearing the endgame there is still something in the wastes that doesn't get rolled over by your high level character.