Another interesting possibility is to make the ongoing damage bump up every failed save. Like the dragon's could start at 5, then 10, then 15... and those quickly get around resistance if you fail saves.
It'd encourage people to do things like make Heal checks for extra saves, to get it off someone![]()
Its an interesting idea, but I think it is more than a little dangerous. In my time I have seen some horrendous runs of (bad luck) failing saves, this would potentially turn minor ongoing damage into massive ongoing damage.
In my opinion the standard resistances from items and armour are fine, strong in the right encounter and useless in all other encounters. I do feel however that racial abilities like "half level +x" are too strong, and potions of resistance are too easy.
I would simply remove potions of resistance from the game and maybe reduce racial resistances and the like to "half level".