I'm not sure what time frame you're thinking of. UA may take delivery of more A319/320s. If Airbus can improve the range of the A321 to cover all of UA's transcon flights, then UA may order some A321s. I've been repeatedly told by the appropriate folks at UA that they won't buy A321s until they can be freely substituted into any A319/A320 rotation (except perhaps ETOPS).

More B777s are a possibility, including perhaps B777-200LR and B777-300ER, the latter to replace B747-400s.

If UA kills TED before TED kills UA, count on UA to order B7E7s to replace the B767-200s for premium transcon service. There is a good chance that UA could be the launch customer. B7E7 would later replace B767-300s in international service and finally the recently ordered B767-300s in domestic service.

I see no chance of UA ordering any Boeing narrow-bodies or Airbus wide-bodies given the current model lineup.