Just recently I read through the post about overgearing, and it reminded me of this old MMORPG I played that I'm sure 95% of you have never heard about(I came by it accidentally nearly a decade ago).
This game had a unique system with gear that solved:
1. Prevented Overgearing new players
2. Allowing BIS Gear Trading
3. Item/gold Sinking
4. Giving Legendary Gear Status
5. Vertical/Horizontal Gear progression
Sounds too good to be true, right?
I'll explain how the system works. Perhaps, with this system combined with the system presented for AoC, might solve even more issues
In this MMORPG I played, you would progress through the levels like a typical game. At a certain point, the player is introduced to open world bossing. Now, you would expect the player to get gear from a boss drop, right? Wrong. The boss dropped an item, which usually new players begin to think as a trash drop, but no. The drop is actually a quest-item. Typically MMORPGs have quest-items untradeable, however, this one didn't follow such rules. Players can either trade this quest-item to other players or they can turn it in to an NPC for a piece of gear. Here is the catch: You must reach a certain milestone(level), to get access to this quest. Every few levels, a new NPC will give a new quest for the next piece of gear for the set. All of these quests were non-repeatable. So, you would either get rich enough in-game to buy-out all the quest-items(very rare), or you simply go bossing to retrieve the quest-items(common method). The gear received from the NPC would always be the BIS.
For the guilds, they would have a bank vault that stored donated quest-items to new players. They never give out those low level quest-items for free, because they had a meritocracy system, even in the non-competitive guilds. All quest-items, regardless of the level you were at, were scarce. You can imagine the challenge of endgame gearing. However, since low level quest-items were still much easier to obtain, new players can still catch up to endgame players relatively quickly, but endgame players were still competing with each other for BIS in the same gear. In other words, there is an endless gear progression, but it keeps slowing down the further you pushed. The last update that added new gear to this game was about 5 years ago. Of the thousands of people that still play this game actively at endgame, there are only about 10 people per server with a complete BIS gear, right now.
You might be wondering to yourself, is this a vertical progression system or a horizontal progression system?
It is both.

Now that I explained the gearing system of this old MMORPG, let's see how we can combine it with the exact profession system of AoC.
Imagine if crafters could make gear that could be used as a quest-item combined with droppable quest-items from the open world bosses, for example, a dragon scale, then turn that it to an NPC for a piece of gear. The quest could be unlocked after reaching a certain milestone(I am not sure what would make a good milestone).
Ex.
Steel Armor QI + Dragon Scale QI = Dragon Armor
This will solve:
1. Not Overgearing new players due to quest milestones
2. Allow gear trading via use of quest-items
3. Item/Gold sink because the items are going back into the game
4. Giving Legendary Gear Status due to scarcity and competition
5. Giving another incentive for players to either progress their node or destroy another node for particular bosses and resources
6. Vertical/Horizontal Gear Progression by combining the two
7. Allowing professions that make gear still feel integral for gear progression
Feel free to point out as many flaws as you can with this system.