Quote from Magnus_Vortex >>

I also like the idea of true strike being used to connect an attack against someone you really struggle to hit. If I can only hit you on a natural 21, having advantage doesn't help me much. Whereas adding my proficiency bonus to the attack roll would be amazing, especially if I can get advantage through other means. Might need to be twice the proficiency bonus, or maybe plus a constant value (e.g. prof+1 or prof+2) - I'm not sure of the right numbers off the top of my head. It wouldn't be in your standard rotation, but could be a niche spell to pull out under the right circumstances.

This is actually how the spell used to function in 3.5, and is how I would go about amending it myself. I agree that gaining advantage doesn't seem all that helpful, but the spell used to provide a +20 to your attack roll, which really made a big difference in certain situations. That said, it was also a 1st level Sorc/Wiz spell, so as a cantrip a + 20 is a obviously too much. Basically, I would grant a bonus to hit and advantage. If keeping it a cantrip, then a small bonus, but if changed to a 1st level spell then I'd probably make it significantly higher as it used to be.

The point of the spell as I understand it, is to help you hit something that is otherwise very difficult to hit, or when it is important not to miss. An example that comes to mind for me is when the archer (I forget his name) in the Hobbit series shoots an arrow at Smog's weak spot, a single missing scale. That kind of called shot is the perfect example of when you would need a True Strike spell.

Part of the problem with true strike is simply that it's a cantrip; an obvious fix would be to make a 1st-level reaction spell when you miss to add a bonus to the roll to potentially turn it into a hit, basically similar to how shield works but for helping you to hit an enemy rather than helping enemies to miss you. But this would then step on the toes of War Cleric, especially if it could also be used to help allies, but you could argue that's fine as the War Cleric still gets it "for free" (Channel Divinity rather than burning a spell slot).

As a cantrip though it's really hard to find a good way to keep true strike both useful, and balanced, because cantrips by their very nature are spammable so their only real cost is action economy.

My version is modeled a bit after witch bolt, which is another spell that needs fixing. After a successful attack roll with a cantrip or weapon, you can TS as a bonus action to begin concentration. If you maintain concentration until your next turn and are still in range, you can repeat the cantrip or weapon attack and it automatically hits. 

Need to work on the wording. No chaining, no crits, no higher-level spells, but it can provide some consistency for those who want it.