United are a very rich club but the chances of the current (or future owners) investing in expanding the stadium are slim. Old Trafford is already the largest football club stadium in the UK by nearly 15,000 seats. Anfield only had 45,000 seats until 2016 which meant demand far exceeded supply making the main stand expansion a great idea (with 6 year payback due to corporate seat revenue). Anfield Road stand rebuild has stalled (although is still very likely) because extra behind goal seating would be general admission only and the payback would be 15+ years. Highbury was a bit less than 40,000 and White Hart Lane 36,000. Man City and West Ham's new stadiums had very different finances for obvious reasons. I don't know United's position in terms of ticket demand, expanding the Bobby Charlton stand over the railway would provide options for both general admission and corporate seatings but do they need it? I very much doubt they could fill 90,000 seats for less desirable premier league or champions league matches, let alone the FA and League cups. The main way to try would be slashing ticket prices which would remove the financial benefit of expanding the stadium.
Although demand has probably dropped off somewhat with the decline in the team, I think it’s still pretty strong. I believe there around 55,000 season ticket holders, which only leaves around 20,000 for ad hoc purchases. League games require joining the supporters club, I can’t recall a league game going to general sale.
There was the expansion in 2006 which added 8,000 seats which was fairly quickly gobbled up. There is also the factor that capacity is about to reduce as seats are converted to add more disabled area.
Expanding Bobby Charlton would supposedly take capacity to 80,000.
https://www.statista.com/statistics/268646/manchester-united-average-per-game-attendance/ says average attendance has been around 75,000 for the last decade, adding another 5,000 doesn’t seem much of a stretch.
Though coming back to the original topic, I wonder if the council would insist on transport improvements as part of any planning permission. It’s already horrible to get away from after a match, more people would only make it worse.