Having two different ONS/WAR gametypes might be a good approach, as ONS was a substantial part of 2k4's community, as far as I understand, but I'm not sure how they'd go about differentiating them enough. To anyone other than the hardcore ONS fans, it'd seem like having Capture the Flag and Capture the Tree.
If they could take the AS aspect of WAR, and maybe introduce some XMP-esque aspects (energy management, locked doors...) to a more prominent level they could better differentiate the two. Including the UC style species differences (that were a latent, but still present part of 2k4) they could simulate XMP's class-based gameplay as well. Some classes might be better at building nodes, others could carry multiple deployables. I certainly wouldn't mind the introduction of some of these elements from the UC series... especially the more refined adrenaline system.
To me (and a lot of the EU ONS community) ONS was about battles up to 8on8. But mainly less players (6on6 or even 3on3 sometimes).
The good maps for that had sizes of Torlan and Dawn (RedPlanet was much too big for example).
And it was purely about connecting nodes*, such as CTF is purely about flags (XMP for example is a different gametype because it adds the "power"-element).
If that was kept for ONS in UT3 then WAR could have been the gametype with the huuuge maps that connect to each other, AS-like-objectives, Orbs, ressource and power management as it was said when it still was named Conquest.
*) see here for how I had like the maps and nodes in UT 3: http://forums.beyondunreal.com/showpost.php?p=2112804&postcount=13