I'm creating a mod and I'm modifying the game's pregame. I'm trying to replace the images with a video I created but so far without success. I would like to know if it's possible to make this change and if anyone can help me.

yeah and the templates exist in the base game, just aren't implemented yet.  We've played around with that too on historical. 

Frankly, for my simple SP gameplay, capturing is quite a convenient and fun strategy as it enables getting a foothold in enemy territory and also get resources  (=new troops) from nearby.

In DelendaEst there is the additional concept of slaves (with a limited lifetime due to the hard life they are forced to endure). Could that be the additional working unit you are looking for?

Do chariots really have to be that fat? Feels like many QoM (Quality of Misery) measures over time, en masse since RC2. Some trees or houses and my heroine isn't where she's supposed to half the time; here she's stuck on a citizen. It's agonizing to frequently watch my champions take the scenic route around a mountain or something...

I think both of you have good points.  And of course, I'm going to encourage you the take a look at historical patch.  We have tried addressing this a little.  As there are now "serf" units which can still fight but not as well as CS, but are better at gathering res.  Then you have the merc type units that can build and fight, but not gather.  And of course you have the champion, elite fighters who cant gather or build.   I like the historical accuracy of the citizen soldier, and the more i read and learn about the different civilizations I agree its an important aspect of 0ad that makes it unique!

Its been talked about before, but citizen soldiers are quite fundamental to 0ad. There's a big discussion here:  Citizen soldiers allow for big battles that distinguish 0ad from aoe2, and they are quite historically accurate which is something 0ad strives to achieve. That being said, they contribute to snowballing, and mainly contribute to the "booming = turtling" phenomenon. I would like to not abandon citizen soldiers entirely, as it makes 0ad unique, but we can increase the importance of non-soldier gatherers by changing gather rates and/or by introducing a new "laborer" unit.