Of course I have no clue as to what this will play out as but let's say there's two kind of general hubs a carrier will operate: 1) an O/D (origin and destination, aka non-stop traffic) and 2) a connecting hub.

While I don't think one can say a hub is purely one or the other, I do think certian ones lean twoards one side or the other.

For example EWR, ORD, and LAX strike me as being closer to the first while IAH, IAD and DEN seem more like the second, with CLE, SFO, NRT falling somewhere in the middle for a combined UA/CO.

If you follow the theory that an airline will want to firstly serve higher yielding O/D traffic before connections then you've got an interesting dynamic at hubs with massive amounts of O/D demand as well as constrained facilities (be it gates, slots, space, etc.) as they'll have to balance how to maximize flights.

Hence a combined CO/UA might want to route connecting passengers away from an already constrained EWR/ORD in order to operate more O/D service from those airports.

In other words if passengers from the northeast need to go to SEA, CO/UA would rather route them through an airport with excess capacity (i.e. IAD, DEN, CLE or even IAH) and preserve the limited resources of ORD or EWR for more point-to-point flying into secondary markets. (Why run an extra MHT-EWR and EWR-SEA flight when they could instead offer a EWR-Europe service with the gates/slots?)

Anyways where all this is going is that I'd guess we'll see IAD, DEN and IAH become the "super" connecting hubs of the northeast, southeast/Latin America, and Western US respectively with ORD's and EWR's limited space & slots focused on going after new O/D markets.

I think SFO will continue to enjoy a comfortable balance somewhere in the middle serving as the gateway for Asia as well as north/south connections along the west coast. Likewise I think CLE will retain flights where there's a strong enough O/D base to support the flight (although this might mean downsizing equipment) and LAX will continue to add non-stop flights to markets that have enough O/D traffic to justify service.

$.02