I think implementing other forms of money sinks into the game that come from osrs should be implemented first to combat the imcominggp into the game.
Examples:
Bonds turning untradeable after trading with a % fee to make them tradeable again.
Degrading armour and weapons, Barrows equipment, Crystal equipment, Abyssal tentcale, etc.
Nightmare zone has also taken a lot of gp out of osrs if that were ever to be implemented here, not that it would be neede for leveling but maybe for the rewards like snapegrass for ironman.
I agree to make Degrading Armour and Weapons, this would make a slight effect and would not be inconvenient.
Nightmare Zone would not be a bad idea. Only I think there should still be like a 1-2% tax fee, because a % that small would not make a significant change, but in the long run would definitely help the economy.
Now this may not be like by the majority but I think you should maybe make the "Well Of Goodwill" 50m rather than 25m, that would also help out a lot.