As bbh 11 has said, it will be managing things like toilets at HT, and exiting at full time - how do you maintain social spacing when everyone is in a rush for a wee before the second half, or to get away after the match.
Noting Luke's comparison with supermarkets, that's valid when moving about but people are then sat in (relatively) close proximity for the best part of 2 hours.
I can see masks being compulsory when moving about. And given all the research data, singing and shouting might be a problem!
25% sounds sensible to get things started, though that will cause all the logistical headaches mentioned by most other posters, deciding which ST holders are allowed in for each game, how households can be accommodated, etc.