I feel like the best way to "get rich quick" would be to set just one goal and stick to it. The first people to access a major item or item source would get rich pretty quickly. But keep in mind there won't be any billionaires in the game or anything, so who knows how "rich" you'll get.
85 slayer: very hard; very obvious
Mage's book/master wand: Best mage weapon/shield. Not too difficult to get. Would get a lot of magic levels along the way. Requires a TON of runes, though.
Antifire potions: farm metal dragons for rune hatchets
Dagannoth Rex: dragon hatchets, berserker rings, warrior rings; easy boss to farm, not many requirements. Just full rune and slayer dart pretty much.
Just gotta think outside the box a little-- which items are taken for granted and will have a very high demand and very low supply like rune hatchets?
Bah, you're on to my cunning MTA plan.
While those two items are the obvious, once the economy picks up, it will be far more profitable to grind Infinity Boots (best Magic stats in the game) as an endgame item.
On the flip side, I think Crafting will be huge, so an obvious is just grinding out 70 or 80 with Flax & Bowstrings.