If Temporary Hit Points exist, then why not Temporary Damage?

[Arkhios]

And for the sake of symmetry, I don't think there's anything in the rules that maxes out how many temp HP you can have at any one time. If the DM creates an item that grants 1d20 x 100 temp HP, you could end up with 2,000. While that might be broken, it's not technically against the rules (as opposed to, say, something that brings you to level 21, or gives you negative HP, or anything else that the system is incompatible with).

I'm still not sure I'm into LTD in this way for a number of reasons. Not the least of which is I don't feel as though all elements must maintain symmetry. Where are our negative levels? Or what's the opposite of a saving throw? Why is 20 a crit but a 1 not some kind of anti-crit?

True, but that's entirely in the hands of each individual DM's, and neither Temporary Hit Point's or LTD's fault. Mechanically the restriction is same for both, and that's the point.

I agree there doesn't have to be symmetry in everything, but this whole topic is purely hypothetical and sprung from a intentionally simplified question. Ideally, should this end up being implemented, the resulting mechanic is satisfying for everyone.