If in a relatively short time Dagon could invade Cyrodiil, what could the other princes have been doing? Is it possible they could have had some interference in Tamriel that the Champion of Cyrodiil wasn't aware of, and if so, could we see their influence in later games?

For example, maybe while everyone was busy focusing on Dagon and his cronies in the main invasion, another prince came in his/her (its? ) full power and began preparation of a secret shrine. I have the concept in my mind, but I really can't wrap it around details. I need more time for imagination.

Feel free to correct, notify, or remind me of anything. I could be missing something obvious that absolutely rescinds my ideas.

Maybe you have some thoughts.

I wouldn't think a Daedric Prince would need to encourage his followers to build a shrine in secret, shrines can be made by mortals and still be conduits to the prince the shrine was built for. Take the Azura shrine in Skyrim which was built by Dunmer refugees for instance.

Hard to say, its not like every Prince would be all that interested in attacking Tamriel (Nor would it be all that fitting) and making it for their own anyway. Except maybe Bal, but that clearly didn't pan out out well for him the first time. Irrelevant at the end of the day since the status quo was restored.

You can argue Umaril was Meridia's answer, but she didn't seem to be deeply invested in his revenge scheme.

That's not exactly what I'm getting at. A Daedra's minimum influence on Nirn is one thing, this would be their full power for at least a small amount of time. They could cause some real damage, or, on the other hand, they could prepare for something bigger. The scary thought is it might have happened, happen, or will happen... anywhere.

A second invasion or conversion from a different means? A Daedric meta-altercation.

Revolution doesn't necessarily have to be the case. Daedra merely like to torment and influence. A perfect opportunity to accomplish this could have been in the Oblivion Crisis.

I'd be interested to see some sort of half-completed Daedric plot in the hinterlands of whatever province we go to next. Obviously with the barriers restored (or as near as), any invasion plot isn't likely to have occurred, but I see what you're saying and like the idea of some other Daedric plot getting some serious traction prior to Mehrunes Dagon's defeat and the restoration of the metaphysical status quo. I suppose whether or not it would be interested to learn about depends entirely on what kind of plot we're talking about; I think we're kind of bored with invasions now, so hopefully there will be something else in the works instead.

That's the good thing about Daedra; invasions aren't necessarily their top priority. Mephala's quest in Oblivion comes to mind, that poor town.