Thracians, Dacians and Illyrians were possible the bearers of this haplogroup
No, that is not significantly possible. This is because I2a-Din came to the Balkans after Celtic R1b-U152. And it is well known that Celts moved to the region after Thracians, Dacians and Illyrians.
So clear conclusion is I2a-Din should not be connected to Thracians etc.
On the other side there is no single piece of argument which would contradict connection between ALL Balkan I2a-Din and Slavic settlement of the Slavs.