Creating your own output styles would be just much much easier if you include simple IF THEN CLAUSEs or better IF THEN ELSE CLAUSEs.
The way you programmed endnote: Field substitutions could be created and managed by our users themselves
but the power of including simple algorithms to templates would open endless possibilites.
If you don’t want give our users to much power (or creative freedom), it would already be great if you add a feature such field substitutions, but keep the fields open.
If the [LIST OF ALL FIELDS TO CHOOSE FROM] is [EMPTY/ NOT EMPTY/ CONTAINS (also to be choosable)], use [LIST OF ALL FIELDS TO CHOOSE FROM]
I would also love to create my own in-text citation templates (additional ones and not only modify the pregiven ones).