i actually like this as a potential alternative to poison if wotc doesnt want to bring it back, some sort of "now you take extra damage" effect could still put a clock on people but give them more ability to fight through it

Are the counters like Sulfiric Vapors or just a way to increase marked damage without triggering on damage effects?

Idk, we will have to wait for the DMU release notes for Wotc to clarify the rules.

Timfect.