Some are quite loud about their feelings on Call of Duty being the same every year. In fact, this year the game succumbed to a huge hate campaign against its Infinite Warfare trailer for once again taking the series to future warfare.
Well, that doesn't mean it's the only thing that has been considered. In fact, Activision heard a lot of radical pitches when the series was at its peak to expand the brand. Perhaps non were quite as crazy as what Vicarious Visions pitched though. GamesRadar have a break down of the idea, that would essentially take the series to Ancient Rome. Activision reportedly liked it, but weren't sure about attaching the brand to such a radical idea, but the dev seemed married to it, and that division appears to be where the project ended.
You can see very early concept stuff in the video here. It is really interesting to see.