You may think it is easy because you have not started to do it
Really, it is a lot of work. But it is not only that, you must know how the engine works to create the events, that is the most difficult part IMHO.
If the team ended burned it was for some reason. Too much work for too few benefit.
Just my guess, I was only part of the team in the very beginning, translating, a few testing and some ideas.
Then real life hit and I had to leave. So I was not part of it when it ended, and do not know the details of why no more scenarios were created and sold as DLC or whatever.
Yes, you need also the graphic part, for the events you may use what is already done because as you see they are rather generic.
Then comes the leaders and the units. Depending on the scenario it may be a problem. I am not a graphic pro, cannot do that

The graphics of the game are very good indeed, specially the units.
A forum member Hobbes, started creating a Aurelian scenario, but did not finish it. If you are
really interested perhaps you may try to finish that one?