I understand the point and was wondering this also for some time.
I do not agree with northern europe being to crowded. And for belgae: why not give them a region and let them control that. I don't know if that is possible script-wise but if it could, make them strong in that area so it is a challenge to overcome those tribes. Most tribes were not interested in expanding anyway. The larger ofcourse did but that's what made them large ;-)
Being dutch I would like to see the Batavs in the game but they are of a far later period. They kicked roman ass along with the canefatus.

Brings me to missing a mod of the period 14 ad till IB(410 AD)


Just my thoughts on this...