British Airways Maintenance Cardiff usually deals with heavy maintenance on all the BA longhaul fleet including the installing of new cabin interriors and in-flight entertainment.
I believe the hangar at Heathrow have been modified to accomodate the A380 which of course may bee required for light maintenance etc. In the long-term there hasn't been anything solid to say that major checks which will be required a few years downline may not be completed at Cardiff. CWL will also remain an alternate so it is a possible for diversionary reasons.
There's several rumours that the B787 will be maintained at Cardiff and also not certain at the moment if the aircraft will be delivered to Cardiff and have the final cabin interior added as have been done with the B773.
A350 as another longhaul aircraft intending to replace the B747 it would make sense if heavy maintenance was performed at Cardiff as there will be room available.
BA shorthaul fleet. There has been talks of the shorthaul fleet moving to South Wales from Glasgow and likely to be in the superhangar at St Athan but with the MoD not intending to hand the hangar to the Welsh Government until 2017 then not sure if this has been cancelled.
Lots of rumours at the moment but very little facts regarding to future maintenance of BA aircraft.