Proposed House Rule: A Fixed DC

I've been gradually coming around to the notion that D&D, and especially 5e, should use a fixed DC for all skill checks. Obviously, this wouldn't include attack rolls, saving throws, or most other combat-related rolls, but for skill checks, and especially non-combat skill checks, I think it's worth considering.

The reason for this is that, looking back, I'm not convinced my assessment of appropriate DCs hasn't been significantly influenced by the capability of the PCs under question - with better PCs facing a higher DC than a less-skilled PC would for the same challenge. In which case, as in much of 4e, you have a whole host of benefits and upgrades, and the associated rules... that then boil down to "roll a 10 or more".

So I'm leaning to a simpler assessment of difficulty, in that the DM would ask the question, "can this be done?" There would be three answers:

Yes, of course! If the task is trivially easy, such that anyone remotely capable could do it, no roll is necessary - success is automatic.
• No, that's impossible. Conversely, if the task is simply not possible, no roll is necessary - success is impossible.
• Maybe. In the third case, roll against a fixed DC of 15.

The big advantages of this is that it's a somewhat more objective judge of difficulty (no modifying DCs to suit), and it's obviously faster than having an extra step of asking the DC. On the other hand, it doesn't have much scope for tasks of different difficulty.

Except...

The rules do have one other tool for affecting difficulty: dis/advantage. If the situation is such that the task is much harder than normal, but not impossible, then disadvantage can be applied. Likewise, if the situation is such that it's much easier than normal, apply advantage. Job done.

(I'm further inclined to think that everything else should be handled by the character's innate abilities, with no "circumstantial bonuses" or the like. Especially since such bonuses don't work at all well with dis/advantage.)

One other thing: I'm inclined to also include some notion of extraordinary success, where the roll hits 20, 25, or higher. For some tasks, notably things like knowledge checks, it may be worth adding graduated results for better rolls. But, even so, I'd leave the basic success level at DC 15, with each step above that giving some additional benefit, rather than changing the DC itself.

(This does also mean that things like knowledge skills should probably be restricted to general "what do I know about...?" questions, rather than specifics - otherwise, those specifics would tie the DM into the fixed DC of 15!)

I'll need to give this one a go to see how it plays before making any solid decision on it, though.