Putting aside all the politics, on a technical level, 25,000 is possible. I was speaking to an architect who is also a city fan, and he said it is feasible, but it would require a walkway adjacent to the railway line to create another access point to Albany Road. The Butts already has planning permission up to 11 or 12K, based on quite modest proposals. Traffic congestion is also a red herring because the vast majority would park in the city centre and therefore access would be dispersed around the ring road. There are plenty of centrally located grounds that can be used as an example. St James Park has only 200 on-site parking spaces.
I don't believe it will happen for lots of reasons. However, lets say a viable, coherent plan is put forward. We would have to decide whether we would back it. Personally, I would, not in support of SISU, but because of the long-term viability of the football club. This goes beyond the financials - and for me is more about club identity and having somewhere to lay our hat and call home (and feel like home). A long term future as Ricoh squatters is a bleak prospect for me. A viable stadium plan that is backed by the fans might also prick the ears of potential investors who might see the Ricoh situation as a massive roadblock. Wishful thinking? Perhaps.
It won't happen, but that will be down to politics and SISUs inability to get anything right more than the technical feasibility imo.