As I understand it, TP and MP are going to be merged into a singular MP bar. Which makes sense, because most jobs tend to only use one or the other.

On DRK, for example, the only TP costs come from your base combo actions. So you don't actually manage TP at all. You could convert these to MP costs under the new system if you really wanted to, but it raises a number of issues. For one, it raises questions like "Should Syphon Strike cost MP, restore MP, or both?" It's also worth remembering that the average player in Heavensward struggled with the MP drain from Darkside. Imagine if every one of your combo actions drained MP. Good luck with that.

I think it's a fairly safe bet that combo actions won't have resource costs attached to them.

On WAR, the only abilities that use TP outside of your combo actions are Tomahawk and Overpower. I suppose one way that you could make Overpower free is if you dropped down the potency significantly relative to Decimate and made it generate gauge. But you'd need to offer something equivalent to the other tanks.

I think PLD is slightly more complex because they're really the only tank that uses both TP and MP. In single target, that shouldn't matter too much, especially if Shield Bash gets dropped. In AoE, though, you're potentially going to have Flash and Total Eclipse at odds with each other, where they weren't previously, unless they either drop costs or move some offensive abilities to the Oath gauge.

There are a few ways of resolving this, but we'll see.