Heritage listed grandstands lack any modern amenities whatsoever to support back of house for professional events. One of the biggest challenges to staging sport is the drainage underlying the Showgrounds, which is located on Brisbane's most flood-affected street. It's not unknown for cars to float down surrounding streets in moderate downpours, and a lot of the residential development of the precinct involves significant under-street stormwater detention.The grandstands on the western side of the oval wouldn't be touched I wouldn't think - they're beautiful buildings. The entire site of the RNA Showgrounds is heritage listed, but that hasn't stopped certain building being knocked down (these have been what were essentially sheds though with little architectural value). What would potentially happen would Machinery Hill (the bench seating on the NE of the oval) would be demolished and the field would extend in that direction.
As for width, the current width is 10 metres wider than Kardinia Park, but I'd imagine there might be scope for a grandstand to extend up and over Gregory Terrace. I dare say the RNA wouldn't mind some kind of structure that could double as an overpass. There is the road tunnel which lies directly underneath at not a great depth, but I don't think works would be of such a scope that that would be an issue.
All in all, I think it's unlikely, but the arena will have some kind of redevelopment for the equestrian stuff for the Olympics, so it's not totally out of the question. The precinct is part of the Bowen Hills Priority Development Area, so there is scope for cutting of red tape here and there.
A major impediment to hosting cricket and footy there will be the event schedule in non-COVID years. RNA is the main music festival venue in Brisbane, and those events aren't complementary with elite sports turf, especially wickets.