Personally I find it very interesting.
But I would go to the end because leaving half of the grain available is as if you do not change anything.
As a result, having a cereal at the beginning of the game will not allow those who spend HUGE gold to have a titanic production from the beginning.
In addition it will make the gameplay king more aggressive, with the need to have a cereal, because the tribute does not bring.
(If the king plays defensive, he will have to show cohesion with his kingdom to kill the natars)
But the fact of adding more NPCs that waste time and resources, I do not really like the "return attacks".
robbers take up a lot of playing time, and someone who is not very present will still have NPC attacks to deal with.
I think we just have to make croper accessible when people will be able to take it, it avoids the rush, it allows everyone to manage a 15c without too much NPC merchant. Conversely if you add attacks, you force the player to stop on the pve, while at the base it is a pvp game. So I do not really understand the approach to boost the gameplay "solo pve" even if it is true that the natars have always been quiet enough.
I imagined some things but we will still take myself for a madman:
Let's imagine delete/replace croppers, but instead there are villages that have the effect of citadel/region on travian legends.
For example introduce a mechanical: if a room is activated on these famous villages(cant be a capital, if 0 population effect desapears), the zone of influence gives a bonus to the entire area. (kingdom border, or only village border)
For example I take a village natars, I activate a room above:
"Fertile Valley Natars" increases grain production by xx% (or decreases consumption) in the area of influence
Finally we can imagine any good bonus pile or already existing, which will be played in a group rather than alone.
In any case it's nice to see some changes in croppers, so strong since Travian 3.6.