Sorry for a double post, but I have some interesting theories about what type the expansion might be. I'll start with the one you all mainly want:
1. "A big, free bonus pack": while obviously this sounds good for people, but think realistically. The "free" part is the main disadvantage: no one works for free. So even this point alone could make it almost a non-realistic option. But there are more. The "big" part is also discouraging: UT3 is giant already, having to download a pack THAT big would be a challenge for a lot of people. Furthermore, people can work for free or almost free on bonus packs, such as Mega Pack; but it's nothing more than a few new maps, several enhanced maps from the community and some new vehicles... and that's probably it. Nothing as serious as a new UI, gametypes or such. And you won't find a single bonus pack that would give you anything more. The closest big BP was Epic BP for UT2003 - it did include a few gametypes - mainly Invasion and Mutant, but they are both based on DeathMatch, so nowhere close to gametypes like Domination and Assault. Furthermore, bonus packs don't include native changes - and without them, you can't do a lot. So this type is almost certainly not an option.
2. "A retail expansion pack": there is a single example in the Unreal series, Return to Na Pali. I haven't heard a single person saying they liked it - it's exactly the opposite, it had a lot of Coop bugs, and was very incompatible with all other versions of the game. And the company that created it, Legend, closed. Not what Epic would like to see in UT3, I guess. Moreover, expansion packs require the customer to have the first game; but if it's changed so much, why bother him? That would really lead to less customers. Also, having a huge game that is replaced by an expansion pack just as big is a waste of HDD and possibly memory & CPU. Of course, there are always alternatives, like both packs in a single pack, such as UnrealGold (which was, by the way, received quite well). So this isn't a plain dismiss option - it's possible. But let's look at the third point.
3. "A whole new game (a la UT2004)": There are even two examples - Unreal Tournament (99) was that to Unreal, and UT2004 was that to UT2003. And what do you think? Both got legendary and world famous! And they're still the most played games from both Unreal engines! Both had new UI (although not very different as both were quite good already); both had fixed netcode (and that was a primary thing why both got out as new games rather than expansions) - you have to agree that UT3 netcode isn't the best around... Furthermore, both had a lot of fixes and updates in native code that allowed the games to run even better than the original ones - that wouldn't be possible in expansions or BPs. And being new games, customers didn't have to bother with getting games they never planned to play. So I think this would be the best option.
Of course, there might be something I missed or didn't consider, but still, these factors are quite enough to prove that the best option, however people wouldn't like the sound of it, is the best for both the company and the customers.