The XC services literally serve the purpose of not requiring passengers from Birmingham, Sheffield and Leeds to have to change trains at York or Newcastle.
I think what would help with XC (and thus its rolling stock choice) is defining what it actually is for. It
was for infrequent direct journeys for people not wanting to change at Birmingham or London (a bit like a rail-borne National Express as-was), but I'm not sure it is now - the increased frequency of service has turned it into something much more like TPE or the Liverpool-Norwich, of basically being a (very) long distance regional express with lots of overlapping short journeys.
Thus regarding stock I think I'd go with reforming Voyagers as suggested, into 7 or 8-car single sets, and where this would cause capacity problems overlay a local service on the top using cheap unwanted EMUs e.g. 350/2s changed to 2+2 seating or Voyagers where there are no wires. The proposed Birmingham Moor St-Oxford service is one such overlay though not likely to be operated by XC. Another would be a second Manchester to Birmingham with an EMU.
But in the land of the realistic, I think it may just be the Voyagers and 170s, possibly adding some of the Avanti ones. Assuming nobody else bids for them so they can be cheap, I'd still do the reforming to 7-8 car, scrapping end cars, because that would save money, allowing one guard and one steward to operate the whole thing, and just the guard and no catering at off peak times. I suspect the 222s are quite likely to go to Scotland, so they probably aren't relevant.
If we're having to accept lots of standing due to the money not being there for longer units overall, it may be worth considering changing to a more open interior (assuming the partitions aren't structural) and adding standbacks to allow more comfortable standing. Perhaps even consider following the 196 and removing the armrests - it doesn't impact comfort like you'd think (as the seats are still spaced) and does make it easier to walk down the narrow aisle without whacking things and to stand there.
Perhaps also consider reducing 1st to 2+2 and so reducing the size of it to add capacity. I'm no fan of 2+2 1st myself, but it would allow an increase in seats, and if done right may still sell.