Alright, this idea might seem crazy, but I still think it would be nice to do something like it. I'm thinking about a special vehicle gametype with large-scale battles that feature a set of normal, some big and a few gigantic vehicles. And by the latter I don't mean Leviathan-like, I mean really big stuff. I know how that sounds, but let me explain:
Have you guys heard of BF2142's Titan mode? I imagine something similar, with the exeption that big vehicles can't be boarded to walk inside them, that won't work with the Unreal engine.
So here's how it works:
Each team gets a base, defended by a big carrier and a battlecruiser which hover above some green plains / a desert / some islands / whatever.
Stationary vehicles in the base (shaped like big teleport chambers or something like that) teleport players aboard the big ships (a simple passenger transfer, not hard to code).

Carriers: When entering one of the landing bay passenger positions, a new aircraft is spawned and the player positioned inside it. Types would fighter-class, bomber-class and corvette-class. The carrier itself doesn't have any weapons, but it heals any vehicle close to it, has an energy shield and a lot of hitpoints.

Battlecruisers: The battlecruisers are huge weapon-platforms, they have passenger positions for various weapons, air-to-ground as well as air-to-air.

Ground vehicles: While the aircrafts are dogfighting in the skies, huge tanks roll across the ground, some of them are fast hovertanks with energy weapons (to fight other tanks), some are armed with flak-style weapons, and a special, Leviathan-sized type carries big, redeemer-like ground-to-air missiles, built to take out carriers and cruisers.

Win-Condition: The objective for each team would be to guide the missile tanks to the enemy carrier to take it down. When a team's carrier is destroyed, the base is vulnerable and can be conquered by ground troops which have to be brought in via drop ships. Hacking the bases core (just stand next to it for 30 seconds or so) will make your team the winner.

Maps for this gametype would be easy to make, since all they need are two bases and a lot of terrain (maybe with some trees and water areas) between them, no need to think about things like ONS-Node setup or Assault-mapflow.

I like the concept. But it would be a lot of work to do all the modeling/coding for this. Also, there are issues with running maps this big. I think it would be easier to rig the capital ships as map objects with player-controlled turrets and fighter bays working similar to AS-Mothership. I think that would be a lot less clumsy than giant vehicles. Also, maps wouldn't have to be big enough to allow the capital ships to maneuver.

Theirs some guy on the Atari fourms call XyX whos making some kind of battleship thingy that you've explained.

Yeah. . .but I still think that capital ships should be level objects rather than vehicles. You could even script them to move around the map.

Plus, having them as level objects would allow you to board enemy ships, something you can't do with vehicles.

He's making pretty much exactly what you explained. He's already got a bomber, a couple of tanks, and a big (but not huge) airship, but I find it kinda useless, it doesn't really have much for weapons. He also has a spider tank, too. Thats pretty cool.

The mod looks pretty promising, so far.

Yeah, I know him quite well. In fact, if you look at his XS website, you'll find my name there, credited for "website assistance". But the problem is, the Hyperion won't come out anytime soon, and the whole XS mod will be a mere vehicle pack (an excellent one, if not the best, but still a vehicle pack), while I'm thinking of a whole gametype on its own. That's actually a pretty big difference and needs special vehicles with special balancing, while a normal UT vehicle pack will always be balanced for use in Onslaught instead.

The maps don't need to be that huge. Think of maps like ONS-Severance or ONS-VolcanoHigh, not bigger. There is a player limit to consider, after all. If the ships would be a part of the level architecture, they'd be invulnerable and stationary, which makes the concept quite useless. I know it would be hard to code, especially the Karma stuff.
Xyx had a lot of trouble with his Hyperion, vehicles that flew into it could push it around as if it was made of paper. With the help of UT2004Addict (who is also known for creating great vehicles) he found a workaround for these issues not long ago, but he claimed that it was a pain in the ass to sort that stuff out.
But the rewards for taking that rocky road would be a very unique gameplay, quite different from what we normally play, and a satisfying feeling of power while piloting one of those behemoths around the map.

You aren't talking about the Cruiser, do you? Not much for weapons? That thing can easily kill a Leviathan! The pilot can launch four seeking missiles (that also act as flares to other seeker missiles like AVRiLs) and can drop Bombs which spawn Unreal Monsters. Each of the 2 passengers gets a weapon that gives teammates all 4 Adrenaline combos at the same time for a few seconds and poisons enemies. And they can switch over to the particle cannon that does massive damage and can also be used to heal nodes, vehicles or even the cruiser itself. Just because it requires a bit of skill and tactics instead of being a one-shot-everything-goes-boom type of vehicle it doesn't mean that it isn't powerful. Not much for weapons, roflmao.

very interesting.. but also very difficult..

I can't really play online, and bots don't seem to use it right, so it makes it kind of hard for me to use. I didn't mean to say that its a bad ship, its just hard to use the skill and tactics required with one person. I really do like the it, though.
I just didn't really think the weapons were very powerful to me, but I'm sure with a full human controlled crew its extremely powerful.
Although I would rather it had regular bombs than monster spawners...

About huge vehicles: due to major karma issues with the unreal engine, very large vehicles start spinning when hit by, lets say, an ordinary rocket or can be pushed around by raptors like they were made of paper.
It would need some major editing of the default vehicle karma for the vehicles to work properly.
That or removing the karma (if that's even possible) and hope there won't be any karma-less vehicles crashing into eachother.

OR you could just make them map objects with controllable turrets & aircraft hangars based on AS-Mothership's code.

Removing Karma won't work, breaks up the game. But tweaking that paper-like behaviour is possible, UT2004Addict solved that problem and already shared the solution with Xyx. UT2004Addict is a very talented coder and quite generous when asked politely for sharing his experience. He helped me out with my PhaseShifter weapon as well.
I'm sure, if the BW team would really consider making huge battleships he will surely help out. Just drop him a PM on the official UT forums.

LEVEL-BASED OBJECTS!!!

Makes the easier to code so you can walk around them, you can have gameplay elements based on their scripted movement (like occasionally getting close enough for infantry to board the opposing ship, no karma issues, much better game stability, a better system for manning turrets, working hangars, there's no end of benefits. The only drawback is that no one gets to steer the thing, and that ain't so bad.

You could even code a gametype around this similar to the BF2142 Titan mode.

Uhh... you can walk around in bigger vehicles. I made a couple of vehicles myself to test it, and I know small ones can be walked around in, and I tried making a big one too, and although I can walk around in it, thats all I could do with it so far...

For turrets, you can make it so you can only enter them when you are close enough to them. It just takes a bit more work make them enter at the right spot, but its still do-able. The only thing I don't know about is if a vehicle can have more than 10 positions for driver/passengers.

Making them level based objects certainly would be easier, but it is possible to make huge vehicles do the same things and more. Which would be better, in my opinion.

Is that Titan thing even steer-able?

Actually, it is. Sorta. Commander tell it where to move. But I'm not sure having pilotable juant flying battleships is a good idea for a UT2K4 game. I'm still attached to my proposition of making them level objects.