I remember @
Akhenderson talking about making grounded PK Fire activate on shield. Comparing frame data, his grounded PK Fire comes out on frame 20 and his aerial PK Fire comes out on frame 15. Best approach to a normal grounded PK Fire (imo) is either run -> shield or jump. It's telegraphed and easy to intercept. As you know, aerial PK Fire has a plethora of uses and doesn't halt his momentum. Adding activation on aerial PK Fire would mean low commitment for using the move, and I think the idea here is that we would like to incorporate using our brains when playing the character instead of harboring on options that become over-centralized.
I think I remember @
Boiko
talking somewhere about EX Fire being subpar because the best defensive option would be to run up and shield. The move comes out on frame 12 (please hmu if that's not accurate because I can't even find where you said this lol) and it's a one frame window. This is actually very similar to Tekken/Soul Calibur's Just Frame inputs, where moves would require to activate on the first available frame.
It's not something that can be forced and therefore would require practice and an understanding of rhythm/timing to execute. As it stands right now, it is an option that recently started picking up in popularity, but it's subpar as it is now (and is often overlooked during commentary, aside the point).
I'm not sure what type of limitations the DT have in coding different properties to the same special move, but I think adding activation on shield on grounded PK Fire wouldn't be too extreme at all. In fact, with the way it is now, it would actually encourage using EX Fire because it would come out 8 frames faster and activate on shield. Pretty rewarding without turning it into a huge over-sight.
Lemme know what y'all think.