Feat: Arcane Gift
Description: The PC gains a spell slot of a level equal to their proficiency bonus in their class with the spellcasting class feature.

The wording was deliberate to avoid warlock spells because that class is incompatible with this feat. But otherwise, I'm trying to think of some powerful combos here. If you were a level 17 arcane trickster or eldritch knight, what level 6 spell would you pick up for maximum fun? Or what if you were a Paladin 16 / Sorcerer 1? Which level 6 sorcerer spell would let you take to set the world on fire?

For a full caster class, not only does this not feel overpowered to me, I'm not sure that it’s even worth taking. And that's fine too.

Tenser's Transformation would be pretty nice on an EK

Quote from TexasDevin >>

Feat: Arcane Gift
Description: The PC gains a spell slot of a level equal to their proficiency bonus in their class with the spellcasting class feature.

The wording was deliberate to avoid warlock spells because that class is incompatible with this feat. But otherwise, I'm trying to think of some powerful combos here. If you were a level 17 arcane trickster or eldritch knight, what level 6 spell would you pick up for maximum fun? Or what if you were a Paladin 16 / Sorcerer 1? Which level 6 sorcerer spell would let you take to set the world on fire?

For a full caster class, not only does this not feel overpowered to me, I'm not sure that it’s even worth taking. And that's fine too.

 
So a Fighter 19 / Wizard 1 would end up with a 6th-level spell slot…. Seems like a problem to me.

So a Wizard 10 / Cleric 10 would end up with a 6th-level Wiz spell slot and a 6th-level Cleric spell slot…. Also seems like a problem to me.

I think I’d like to see the next draft of this feat.

Quote from IamSposta >> 

So a Fighter 19 / Wizard 1 would end up with a 6th-level spell slot…. Seems like a problem to me.

Thanks for your feedback! This is great food for thought and exactly the kind of stress test I wanted to apply to my concept. If you had the option, which 6th level wizard spell would you pick to really pump up the power on your 20th level character?

Fighter 19 / Wizard 1 looks tricky to me because you would have to pick up the feat before you got your wizard dip, or else you would be limited to picking two level 1 wizard spells and would have to rely on copying higher level spells into your spell book after you took the feat. If I was Fighter 19 / Wizard 1, I think I would pick shield and Tenser's transformation as my spells (good call scatterbraind). Tenser's would still be fairly useful, even if the last three bullet points are redundant to a level 19 fighter. The extra force damage for one or two encounters is quite nice, but at a pretty high cost of a feat and a level dip. Globe of invulnerability wouldn't suck for a melee player either. I would definitely plan the spell in coordination with action surge.

Your example highlights an unintended oversight in my wording. You are only meant to get a single slot with this feat, although in your wizard/cleric case, you could use it to cast one wizard spell or a cleric spell per long rest. Let me try to word it a little better:

The PC gains one spell slot of a level equal to their proficiency bonus. With it, they can cast a spell from the spell list of their class with the spellcasting class feature.

Try something more like this instead:

Additional Spell Slot (feat)

Prerequisite: the Spellcasting feature.

You gain one spell slot of a level equal to your proficiency bonus with which to cast any spell that you can of an appropriate level.

That's funny. I actually had that exact prerequisite in my original draft, but I took it out when I thought the feat's description made it redundant. Easy enough to add it back.

Yeah, I left it out of the description instead.