Personally i think one of the main reasons why they havent implemented it yet (apart from time constraints) is the economical impact a sudden flush of new precursors would have on the items that are used to craft the other legendary gifts, most notably t6 common, rare and fine mats, silver doubloons, etc.
I play since head start and got my first (and only) Legendary, Juggernaut, in about 7 months. For me it was a fantastic journey but back in those days, the economy and players wasnt as focussed on precursors and legendaries as it seems now and the wealth gap wasnt as high. Right now it seems a daunting task to do for new players and even though i have lots of gold (or assets to sell) now, i stopped working on crafting my 2nd legendary after the wardrobe changes because in my eyes, they arent worth it anymore because their novelty has worn off.
Here is my take:
I dont think it makes sense to implement it for the old pres/leggies, we need a whole new system and new legendaries, that require new mats so everybody more or less starts on the same page. Leave the old pres/leggies how they are.
In general, the new legendaries should be account bound and their acquisition should require alot of completed achievements from all over the game rather than alot of mats you have to farm or buy with gold.
The precursor should take a couple of months to acquire and should be limited per account, a good way to do this would be to award it, if you completed all achievements of each Living Story Episode per Season. That would be 2 per year and this way, Anet makes sure that players play the game on a regular basis.
They should focus more on achievements from all over the game rather than gw2 gold eu, so lets see what we got:
WvW: A token for completing the tournament meta achievement, also 2 per year.
Dungens: Dungeon Master title Token. OF course this presents the problem that it can only be done once, so twice a year, this achievement will be reset, without losing the title or the already claimed achievement points. After the reset, you have to run each Dungeon Path once again to get the token, maybe reward 5 AP for completing a whole dungeon to make up for the frequent dungeon runner repeatable achievement.
Map Completion: Should be required for new legendaries as well.
Personal Story: Awards a Token for each character
PvP: 2 Legendary Reward Tracks per Year that award a token each
World Bosses: Kill each World Boss and free each temple once for a token, also resets twice a year
Jumping Puzzles: Complete each JP for a token, resets twice a year
Fractals: Reach rank 50 for a token, resets twice a year
Crafting: The gifts are crafted at lvl 500, so you will need 2 specific crafts at lvl 500 depending on the legendary you craft.
Monthly: A token for each monthly Achievement completed, you will need 5-6 per Legendary
Thats about what i think should be put achievement wise into forging a new legendary. It requires alot of work but i think everything is manageable within 6 months and its timegated for twice a year. Of course, depending on your favourite playstyle, there are some things some players will absolutely hate to get the token from. To mitigate that a little bit, you could be awarded a token of your choice from achievement chests, for every 2.5k or 5k AP accumulated.
Should there be tradeable mats involved? The only reason why tradeable goods should be included in forging a legendary is because Anet makes money with selling gems. The process of acquiring all tokens will require players to log in on a general basis, which is good for Anet. If they want to implement a gold sink into the process, I would make it a gem store item that costs a couple of thousand gems. That way, players who rather spend real money on the game, can buy it directly for a price that doesnt change and players who rather earn gold in game can do that as well. The rising gem/gold exchange rate takes inflation into consideration.