One time, I tried making a Counter-Spell card (as in a Counter-Trap, but a Spell version of it), even though no such type of cards exist... it didn't seem to do anything... and it seems turning Solemn Judgment into a Counter-Spell makes it unable to activate its' effect. What would I need to do to create a Counter-Spell that'll work?
You would need to go check the core itself, because a lot of stuff are hardcoded in it, and update it yourself

A simple exemple would be Field Trap, We had to update the core so the card is set in the Field zone instead of the spell/trap zone