There are some really great ideas in this thread. Crafting is definitely a system that has much potential and I hope that this update will be able to bring it back to life and keep it alive.

All the professions
With alts, one can already easily cover all professions and never need to trade with another player. Adding a fourth profession doesn't add anything more than convenience. Allowing players to have all professions would be granting even more convenience. If it must be so, additional professions could be gated by unlocking each new profession for gold, mithril, guild tokens or perhaps progression on already learned crafts. And as others suggested, progression on profession 4+ could be inhibited by diminished returns on craft XP if there are worries of a disproportionate advantage.

More reward tracks
Crafting should have its own reward track(s) with rewards such as ingredient packs, craft XP boosts, and picker boxes for resources, special ingredients, rare ingredients and guild tokens. Maybe there could be unique titles and cosmetics (did someone say tool auras?). These could be seasonal like the LI tracks, or maybe unlocked by progressing to a certain level in a crafting guild, or bartered from the guild which would disappear once completed forcing you to barter for a new one. Possible variations include having a unique track for each guild, having tracks be character-specific, and allowing only one reward track to be active (earning XP) at a time. Side suggestion: If crafting can get reward tracks, so should VIPs for boosts and similar.

Crafted gear progression
To keep crafting relevant at end-game, crafted gear should have some minor progression similar to how LIs are upgraded every so often during the end-game cycle. You could trade it in along with some tokens crafted by a crafter depending on the type of gear/jewellery to enhance the item level. It would have to be at an NPC since the gear can't be traded to a crafter once equipped. The original recipe could also get an update, but the upgrade should always be cheaper than a new piece. Of course, crafted gear should still be weaker than instance and raid gear but it should also keep up with the power creep so that newly levelled characters aren't too far behind.

New profession
Essence crafting was shoe-horned into the other crafts as a quick fix to make crafting relevant again. It's perhaps time to make essence-crafting its own profession (Artificer?). I understand there would be some backlash if everyone suddenly lost the ability to craft essences and had to start over crafting low-level essences. It might be that essence crafting in non-artificer professions gets removed after a longer period so that people have time to level their artificer job.
The artificer should also have a way to craft Universal Solvents, Essence and Tracery Removal Scrolls, even if on a long cooldown.
This new profession could also include combining LI enhancement runes into higher quality (not tier) versions. Runes should also be disenchantable into shards (remember those?) which would be used in the enhancement rune enhancement process. Just don't go berserk with the higher tier combination costs like in the old LI system.

We need to talk about the gathering guilds
Others have mentioned this but I want to emphasize that it is hard to imagine the possible benefits for joining the new gathering guilds. What are the plans for these? I'm sure there are already ideas floating around, but some suggestions include access to fast travel to a guild hall (nicely decorated (@Scenario)), temporary gathering buffs (+chance for bonus resources, critical ingredients, +wider node tracking radius) and cosmetic tools.

Additional quality of life changes
  • Make it easier to gather low-level resources by adding local quests, such as "kill 10 mobs in a certain region for 20 resources". Maybe something to add to the gathering guilds? These could also be task-style quests.
  • Give us the opportunity to use Mithril Coins to complete craft levels, maybe even for guild reputation.
  • Make craft XP bonuses (both special event XP and bonuses from items) work on Forester, Prospector and Farmer.
  • Add craft boosts to Valar packages.
  • Fast travel to crafting guilds