Let me supplement something D L X said. From what I've read, you're not going to see a bigger version of UAL. Take the UAL route map and drop the US route map right on top of it. What routes do you see in common?
I'm sure there are overlapping flights on MetroJet vs. UAL mainline out of IAD too, but not too many. If you consider a combined NYC, there are some overlapping flights there, too. Again, not that many.
My point here is that UAL is reaching critical mass at ORD with capacity. One of the reasons that UAL found US so attractive is that they can complement their own route structure with what US already has in place.
I expect to see some more intra-hub routes added or upgraded to a wide-body aircraft. Specifically, DEN-CLT, PIT-DEN, LAX-PIT, and SFO-PIT.
As of today, US has six A330s in service with a seventh being delivered this month. Contractually, US/UA is required to take delivery of all twelve A330s. Will they keep them? Dunno.
I do know this. UAL does operate a seriously impressive number of 757/767/777 aircraft. Add in the 66 757/767 aircraft that US operates, and it looks better. Does that make the A330 an oddball? Yup. However, it becomes less of an oddball if United buys more of them. Would they do that? It wouldn't surprise me if they did. Remember, fleet commonality makes sense to smaller carriers and carriers with relatively few maintenance bases. However, a combined US/UAL has at least seven domestic maintenance facilities (US: PIT (B737s, DC9s, and MD80s), PHL (A330), LAX (757/767), CLT (737/757/767/F100) and UAL: ORD, IAD, SFO, DEN, LAX).
One thing that varies in the operations of US and UAL is their flight attendant dispatching. A UAL FA can be on a different type of aircraft on every leg. However, most US FAs stick to the same type of aircraft for most of their trip. They do switch back and forth between the A319/A320 and the B733 and B734.
US has already told its flight attendants that there would not by any transfers to UAL domiciles for at least two years. Combining existing domiciles would have to come first. That's the NYC, WAS, and PHL bases.
You know, merging the two fleets will be one of the easiest tasks of the merger. Pilot training isn't a big deal. Flight attendant training is a long process, but not really that complicated. Training ramp and baggage crews isn't a problem.
The real problem will be gate and ticketing agents. These poor folks are just now getting used to Sabre. Now, their world is going to change yet again when it goes to Apollo. That brought PIT/PHL/CLT to its knees when US transitioned to Sabre. I think this will be just as bad. Moving to a new CRS is really complicated, and its going to be a logistical nightmare.