You've pretty much answered your own question really. Sunderland and Middlesbrough are close geographically and will be a large crowd. West Bromwich is basically a suburb of Birmingham and again will involve large movements of people who won't necessary get along too well. The same with Leicester and Stoke but not as close geographically.
Millwall are er, just Millwall. And there was some fair level trouble between themselves and Hull at an FA Cup game a few years ago which has stoked fires.
Just something to be wary of in terms of kick off times being changed but I reckon the two games in London will stick to 3pm on the Saturday unless Sky want to show them live.