segregation at matches is 2 lines of seats at most or sometimes just an aisle so a mute arguement.
also if you give an end and a side to each team neither set of fans would be in the same stand.
Take it away from wembley and you will bring back more match goers who go to home and many aways back to going to the semis I don't know any SC holder going out of choice/financial costs even, all would be going if it was at villa park, everton or anfield.
Capacity? well seing many cannot be arsed attending the rounds then I expect those who do bother will more than fill a lower capacity and they are the most deserving having put in the effort, alomg with that with villa as the lowest (but most convenient) there is enough high capacity grounds to suit south v south, north v north and south v north semi finals.
Bar Villa every stadium below could accomodate 23000 fans for each team minimum
North:
Midlands:
South: