Couldn’t SWR have refurbished some of the 450s to 2+2 seating instead of trying to reinvent the wheel unsuccessfully with the 442s and latterly, the 458s.
One solution would be to unbolt the seats and move them between units.
Basically use the 2+2 seats from 458 in the subset of 450s. There has been speculation about some of the 450s becoming BEMUs for Salisbury (and possibly Exeter) services releasing aging 159s
Battery life is generally considered to be around 15-17 years, so creating a subset of 450s with batteries, and seats from 458s would potentially be a much more sensible project as got a remaining lifespan of around 15 years.
However if you want a more wacky idea, then exchanging some more of windows for ones with opening hoppers, and using them as rail tour and excursion (or even footex) hauled stock would be good. The windows are much taller than the older air conditioned mk2s that aren't that aren very popular. Might need a battery power car (but there are a few spare 458 vehicles around that could be used), and ending up with a 9 or 10 car set that is to C1 gauge could be very useful.