I'm not biased, I've literally played every single Unreal franchise game -- also, since the announcement, I've been playing them all quite a bit in rotation -- '99 -> 2K4 -> III, below is how I feel about all three NOW, no nostalgia goggles on.
'99 is the best game in the series and UT3 has the best balance. The 2K series screwed the pooch/jumped the shark, especially so by adding in Onslaught.
Onslaught is NOT Unreal. It's Halo/Battlefield. It's not an Arena based game, and since the weapons are designed around being trapped in an Arena, it's an awful game, it's a whole bunch of people running around with the Lightning gun/shock rifle, hitscanning you to death over huge distances... It's not fun.
Now I'm not against vehicles outright, I'm more against the entire game mode of ONS/Warfare.
Unreal Tournament and Unreal Tournament 2003 both have Assault mode... this mode is the one that should be fleshed out while ONS gets clipped.
Vehicles can exist in Assault mode, just as long as they are only on some maps, and only for certain phases of the attack.... Like "Breach the fortress" -- okay, so we roll up in vehicles until we get inside, then the vehicles are done.
2K's scaling was all messed up, the people too tiny because the maps too large, the movement speed on the tiny people too high, and the ability to change directions mid air with the double jump gave too many jukes. Jumping in an Arena game is supposed to be used tactically, being airborne is bad because there should be a rocket at your landing, also -- the higher your are, the easier you are to pick off with the LG/Sniper/Railgun. Allowing people to just change directions mid air ruins the entire delicate balance of the game and raises the skill ceiling beyond "Pro" to "Robot" levels.
The other, not so obvious, but at the same time still glaring issue with 2K4, is that -- the added movement options, the extra jukes, destroy your ability to play online. At a ping of 100, you almost can't hit anybody with anything, because you have to lead your target, but how do you lead a target that can change direction at ALL times in angles as extreme as 90º/180º at will in all scenarios -- when they are in the air, by a wall, on the ground, nothing stops you from moving right and then going left, even in the air. There is no conservation of momentum, therefor, no reliable way to predict opponent movement, so ping advantage is the absolute strongest in the 2K series... This is a huge problem now, since most of the active servers ping over 100 for me, and I'm based out of Florida.
The only way "Double" Jump could work, is if you didn't have air control on the second jump. The only way it could work, is if when you initiated the second jump, you kept the momentum you had prior to the second jump. Being able to jump forward, and then use your second jump, to jump backwards passed your starting point ruins your ability to predict movement, which is by and large a SUPER HUGE part of what makes arena games successful. Also, double jump height would have to be reduced, because the second side effect of the double jump, invalidates the use of rockets and other splash damage weapons by allowing you to extremely dampen the damage by jumping out of it's splash radius. This is why double jumping should be removed, or replaced by a powerup like the jump boots. [In UT4, we could have Jump Boots and Super Jump Boots, where normal jump boots, give you 6 double jumps and super jump boots give you 3 super jumps.]
While the "Double Jump" and "Dodge Long Jump" are "cool" the damage they actually do to the weapon balance and your ability to predict player movement is waaaaaaay too severe, this is the reason why 2K only survives by ONS and Invasion. Because ONS has gigantic maps and being on foot is generally a death sentence since the vehicles are so strong, hitscan long range weapons like LG and Shock are dominant here when on foot, and over larger distances, it's easier to acquire your target/predict his movement, since he's generally focused on getting from A --> B and invasion because it was Coop, and you weren't shooting at other players that had these abilities. As much as the 2K only community is vocal, they are indeed the minority as all the '99 players and most of the III players agree that the double jump/long jump is what destroyed the continuity of the game.
Also, if we look at all "Successful" arena games, they all have one thing in common, conservation of momentum. From Tribes --> Quake --> Unreal Tournament '99 --> CPMA they all have that one thing in common, you carry your momentum and cannot change on a dime. While CPMA gave you a metric ton of air control, you still had to maintain a vector. This allows opponents the ability to predict your movement. Same with Tribes, you had to maintain your vector, despite flying around with jetpacks, you couldn't be moving forward at 100mph and suddenly decide you wanted to move in the exact opposite direction and just bounce back on a dime, you'd come to a stop and then have to build your speed from scratch with a slow acceleration curve. Same with CPMA. And Quake, while it allowed you to move really fast, you couldn't change directions on a dime without doing a "trick" jump, which caused a hefty amount of damage to yourself, much like impact hammer jumping in UT. This is why UT '99 has the best foundation, because at it's core, it KNEW it wanted to be an Arena game -- the Dodge and Alt fire were it's gimmicks over quake, and even though the dodge gave you a juke, it was still VERY possible to predict player movement, because once you initiated a dodge, you were STUCK on your vector until you landed, same thing with Jumping, as long as you were airbound, you were predictable. In 2K, I could be running straight at you, Jump into the air, and then woopsie, jump again to the right, and literally change my vector by 90º instantaneously. You could also jump forwards, and then backwards, or forward and right, to back, you had waaaaay too much control and being in the air was no longer a deficit, but the SAFEST place to be, which is exactly 100% opposite of the very core that makes an arena game work... this invalidated every weapon that wasn't hitscan, and even made sniper shots virtually impossible to predict, the shots that land, are mostly luck that you guessed right than skill proving that you could aim. This also made the game exponentially harder the higher your ping was, because having a high ping = a delay, which means you have to LEAD targets, leading targets is impossible if they can change their vector at will.
Wall Dodging was a SOLID gameplay mechanic and an overall improvement to the base game, it was only situationally useful and still left you vulnerable to prediction provided it didn't catch your opponent off guard.
So in short, I don't care if you EFFIN LOVED 2k4, the design is broken logically, and if they choose this route as the base game [double jumps/dodge long jumping/ignoring the conservation of momentum] UT4 will be dead before it ever starts.