If they work in an underlying plot in a monster of the week concept it can work, cause to be honest now that I look back at Season 4 it would have worked much better if there wasnt so much melodrama and extended plot device.
Like for example, make it so that W&H are using the Angel Investigations team for their apocalypse, each of them are filling important parts and various things and missions they are given which involves monsters or problems of the week in the end tie together in the apocalypse which is the season finales 2 parter, it ends with part 1 and S6 would begin with part 2.(I just love those cliffhangers).