Right, here is an illustration of the problem, I've never won any prizes for Art.
Dark blue are provinces Macedon owns or will soon conquer.
Light blue are provinces adjacent to those - Generation 1 colonies.
Hatched light blue are adjacent to these, and are Generation 2 colonies.
White dots are Generation 3, white marks are Generation 4.
Dacia has a green background.
You can see that a huge amount of territory is Generation 1. By Generation 4 you are touching the Permanent Terra Incognita and are 2 provinces away from the Baltic. This is why Macedon finds it very easy to colonise a huge amount of eastern Europe.
My proposal would be more like this. I don't think it plays too fast and loose with the geographical boundaries of the area.
The effect is to markedly reduce the number of Generation 1 colonies, and hence increase the number of generations needed to reach further inland, as illustrated thus.
The reason I'm proposing this is because much of my own modding has been aimed at reducing colony spam, and it struck me that actually the map was very important to doing this in the long term. I suspected that also there would be relatively few good ideas for what to do in Eastern Europe!