It appears that after McG is done with his exploration of the future with the upcoming Terminator Salvation, he's planning to go the complete opposite direction and do a historical movie entitled simply Medieval.
Very little (and I don't use that word lightly) has been released about this project yet (including even a basic plot outline) but what we do know is according to Production Weekly a spec script has been written jointly by Mike Finch and Alex Litvak (yeah, don't worry they haven't done anything notable before so you're not on your own having never heard of them).
Having very little to go on I will say that it sounds like an interesting idea for McG to take the completely flip-sided route and explore the past after he's done with Terminator Salvation. I wonder whether he will make this thing with a big budget (which will be most likely because he's probably gotten very comfortable with the freedom of the $200 million budget of Terminator Salvation) or whether he'll aim for a much smaller film - it will certainly be interesting to see which route he takes.
Since there's no word on even a plot I can't say whether the project sounds good or not but McG attempting something like this is at least attention grabbing news.
More news on this as it gets released.
Source: Production Weekly and SlashFilm