TLDR - Incorporate T0's into higher stage regions
The team has given so much love and attention into these assets that I don't see why we shouldn't be utilizing them more.
This increases the illuvial density in the map and the rewarding sensation of finding T1+ illuvials.
It also helps blending the T0's with the rest of the illuvials, gives more meaning to the scanning functionality and increase the value of higher stage runs. Not to mention that having T0's in higher stage regions will help making stronger enemy teams overall with their class/affinity bonuses.
Currently T0's are in almost every meta arena team, not having them mixed in higher stages just forces us to travel to stage 0 regions in order to catch these. Higher stage T0's could also have a higher drop chance at higher stage regions.
Suggestion for the % chance to find a T0 illuvial per region stage:
stage 0 encounters - 100%
stage 1 encounters - 50%
stage 2 encounters - 25%
stage 3 encounters - 0%