I know that the UAs have suggested that Artificer would be an Expert...but I kind of prefer them associated with the Mage group. I prefer the Mage-based epic feats to the Expert ones for Artificer, and I don't like the idea of the Artificer being hedged out of options when they're caster-like. And, well, while I don't have an issue with Expertise on tools, we now have Expertise on skills only, and I don't like the idea of Artificers going around as Experts in palming objects. Artificers should have tools that make them really good at palming objects instead.
Starting proficencies... I think starting proficencies, HP, and multiclass rules are fine.
Level 1. Spellcasting, Tool Expertise, Magical Tinkering. Spellcasting is, well, spellcasting. Rather than being purely Arcane, I would like to see the Artificer share a spell list with the Bard which touches on spells from all three other lists (henceforth the B&A list). And Magical Tinkering is just... Prestigitation/Thaumaturgy/Druidcraft for Artificers. Should honestly be a cantrip like the other three, but for the B&A list. It works really well for bards too.
Just spellcasting isn't enough for a level 1 half-caster. If we're going to be an awesome gish, we need more. So, lets start by pulling Tool Expertise down from the old level 6. And we're going to start breaking tools down into a couple categories. Smithing + Jeweler, Thief + tinker tools, Alchemist+poisoner+brewer+herbalist tools, carpenter+woodcarving, weaver+cobbler+leatherworker, mason+potter+sculpting, calligraphy+painting+cartography. Each group not only gives you double proficency for each member of the group, but other bonuses as well related to their theme. It should be something on par with the warlock's Pact Boon, imho, and synergize with subclasses that rely on things like heavy armor (smith), or Artilerist with lots of gun-like wands (woodworking). You can even make it "cantrip like" if you really really need to by letting them be imbued with your Spark, all Girl Genius lilke.
Right Tool for the Job should probably also merge in above.
Level 2. Imbuements. I'd like to see just a blanket "you can make and use all items" that intersects with good crafting rules. I'd also like to see rules for some consumables as Imbuements. But, yeah. In general, this is good.
Level 3 subclass. Alchemist, Artilerist, Armorer and Battle Smith to start with. After that, I'd like to see a "PaperMaster" class, something closer to thread-based abilities, and weapon-based "unlimited blade works" type thing.
Level 4 feat. 'nough said.
Level 5 is a bit odd, in that the major subclass boost was here before. But now that's been pushed back a level to 6. So, I'm putting Improved Tool Expertise here, in that it can serve to grant Extra Attack or whatever is necessary to keep the power curve on single classes going.
Level 6 is subclass. Level 7 is Flash of Genius, as before. Level 8 is feat. Level 9 is Magic Item Adept. Level 10 is subclass. Phew!
I don't really play past 10, so I'm a bad judge at how the artificer works past this point. But, I'm going to asume its roughly a good power curve and just go with the rest as close as possible; tweak via infusions as needed. Level 11 is Spell Storing Item, 12 is feat, 13 is Magic Item Savaant, 14 is subclass, 15 unlocks the best imbuements, 16 is feat, 17 is Magic Item Savant, 18 is Soul of Artifice, 19 is feat, 20 is epic boon.
Aaannnnddddd... tada. What I would do with the artificer.