mellored
Legend
Not being able to do it all the time is already modeld with the d20 roll.As for the proficiency bonus: The point would be that you're just trying to brute force the effect, when you're not in a position to apply your full skill and power to it. Getting someone into exactly the right spot to pull off one of those tricks to full effect takes effort and luck, and not something you can pull off every single time, which is what's represented in the superiority dice limitation.
I mean, if you try and trip, but roll a 1, then he wasn't in the exact right position. But there's no reason you couldn't try to pull it off every time.
Adding your proficency bonus means you are better at getting them into that position. Not that they will always be there.
If it was "you automaticly trip someone", then yea, you need a limit.
That said, possibly a better model might be...
"Before you attempt to trip someone, you can roll a d20. You can then decide whether to use that roll for the trip, or just make a regular attack, rolling a new die."
Thus, you only try and trip when you think you have a good chance at succeding.