We'll find out on the 21st won't we. Its been a flashpoint in the past.
I'll have a sportsman's bet with you that the only nastiness on the 21st will be obscene chanting and gesticulations between rival fans from a safe distance.
The police will have this one tied down. 500 Ox fans are coming on a special train (remember them?). No alcohol allowed, BTP on board, police escort from station to Merlin or even straight to the ground. All other scheduled trains will be met at Swindon station and Ox fans herded to wherever they can be supervised, or kettled at the station until there are enough of them to move en masse.
Swindon fans will be dissuaded from grouping in numbers by being moved along by police, and if they do they will suffer the same kettling process as Ox fans.
Every pub in central Swindon will have a police presence - possibly combined Wiltshire / TVP (cos they hate us) - and nothing will kick off under CCTV anyway.
There might be the odd opportunist scrap, but they'll soon be sat on. There has been talk of "organised trouble", but for reasons above that won't be in Swindon itself, if at all.
This will be a larger scale version of Oxford going to Luton in the BSP a few months after unsavoury scenes at Oxford-Luton at the Kassam. Plenty of aggressive atmosphere, but police definitely on top.