More Heathrow capacity?
I think you may be getting too hopeful with more LHR capacity. My theory is using A320 instead of A319 on services giving 24 more seats. Operationally this is really easy to do. We will know more when post IATA conference Summer schedule is released in December.
757s haven't been scheduled regularly from LHR for a while. Once LHR T5 opens, the 757s will be based over at T3 for flights to Spain and Italy. Before T5 there may be a chance, but remember the LHR gate at NCL with the airbridge cannot accomodate a 757 since the stands were remarked. It would need to use the current "LGW" gate which isn't ideal. The reason for very rare BA767 visits is that they are a common crew fleet with 757. So if 757 crews are rostered on a NCL and the 757 goes tech, it is possible the only spare may be a 767. Sometimes this happens on the day and very rarely is planned earlier due to maintenance schedules.
Regarding A321, they are based at T4 and may be a possibility to NCL once T5 opens. The visits last year we due to maintenance. All BA Airbuses have heavy maintence as GLA. So once in a while they need to get from LHR. One way to do this was to schedule them on Domestic flights which ended in GLA to get them there and started in GLA to get them back to LHR. As it happened the aircraft worked a full day of domestic services as part of this clever positioning which resulted in visits to both MAN and NCL. This was planned well in advance, and isn't this winter...
CB