I think, there is better chance for AA to share with BA in a NEWLY BUILT terminal 3 for all USA departures. The following shuffle can happen:
1, Move all BA's close allies in oneworld to T5, that includes RJ, AY, IB, JL and possibly future CZ and QR.
2, Rebuild the T3 to the T2 standard and with an additional satellite opening between T3 and T5, the T3B
3, Move all BA's USA operations to the new T3 and share it with AA.
4, Open connectors between T5C and new T3. So that aircraft can still park in T5C while using the facilities in new T3.

I think T3 is still quicker to reach central London. And Heathrow does have plan to rebuild T3. The problem for AA to move to T5 is that AA has too many flights. It is much better to move the smaller oneworld airlines to T5 first and leave room for BA and AA to have its transatlantic terminal in a newly designed and purposely built T3.

T5 is nice but there are many things are showing its lack of space and lack or respect for large volume of transit passengers. The better way to tackle all the problems in T5 now and take advantage of the space can be utilised, is to design a new T3 from a clean sheet of paper and try to consolidate AA and BA's transatlantic routes from there. Plus it is unlikely DL and VS would move out from T3 at this moment. It could be a truly Transatlantic terminal for AA+BA and DL+VS. Ultimately, it would be better for all oneworld airlines to move to T5 and have BA to design a new T3 with its experiences of T5.