As I understand, aggrocolor has been reworked with blizz's api to give a true aggro indicator. I was using the original tag as a targeted indicator for predictive healing and pvp. Can there be another tag that uses the old mechanic?

Yes, this change has made healing difficult for prediction because the threat table isn't updating as fast as the simple Target/Target check from the prior version. So you don't know who will get hit just from raid frames until it's possibly too late.

The targettarget check has been re-added under squares in v4000. I will check if tags get it aswell at a later point.