I'm confused why can't you use it with Warcaster, and how could you possibly use it with Spell Sniper?
Edit: Ok I can see that Spell Sniper doubles range...but surely any sensible GM would rule that because it's a melee attack you would need a reach weapon to increase the range. I thought this was meant to be the edition where natural language and common sense ruled the day.
I'm not really sure I see the issue with Warcaster. But in any case, if they don't want players to use melee cantrips for opportunity attacks they should write in the spell "this may not be used for opportunity attacks". Failing that I'll make a decision if it comes up without the need to legalistically parse langauge.
In any case, all martial characters need some kind of scaling for Opportunity attacks. Maybe a flat D8 per tier is the way to do it?