Hearthpwn

Overall, you have a ton of cards that generate more cards or give more value. And... that's it. I quickly looked through your other sets and saw more card and value generation, but never an endgame. What do you *do* with all those cards? Make combos? Trigger a big effect? Or just overrun your opponent with value and survive fatigue? I feel like your class is lacking a big payoff card.

Denkeeper Sinthi - How often will you have 9 cards in hand? And how often will that ninth card be worth a significant cost reduction. This card is too inflexible and its poor stats don't help. 

The Mnemstone - Does this copy multiple Decay effects from multiple weapons? I assume not, since no other keyword stacks with itself, but want to be sure.

Shrill Banshee, Cursed Construct, Scrappy Protector - Just like Denkeeper Sinthi, these are so specific and have poor stats on-curve. They have big effects if you can trigger them, but you'll never trigger Cursed Construct on turn 2. 

If you are worried about not having good enough Alchemist cards, keep working on them! Don't stop iterating and updating until it's time to post! 

You only have a couple tribe based cards. Kobolds has Dragons and maybe Elementals. Witchwood has Beasts and Murlocs. Maybe add another cross-tribe minion or two between the two sets.

The updated hero power of two 1-cost minions makes the most sense for your class. 3-cost is interesting, but you have multiple cards that deal with 1-cost minions, and changing that breaks your themes. This also makes Flying Witch more interesting.

Job Opening - I agree that it's odd that it's clearly Dr. Boom art, but whatever. I wonder if a different name might be more on-theme, like 'Looking for Group' or something.

Lord Walden - Doesn't that give your minions +4/+4? Seems way too good imo.

From Below, Skilled Grafter, and Chromatic Potion are some of the stronger cards from your previous collection.

Sealed Tome - I don't like a spell with two such different effects. If it summoned a 4/4 or two 2/2s, that's better. But with this, sometimes you want a minion and instead get spells you can't use. I agree that another treasure chest (mimic or not) would be more on-theme.

Raeth Ghostsong - Please tell me your class won't have any Defile cards. Even Wild Pyromancer can be trouble with that effect. 

Other cards look good.

Dark Matter is terrifying. You have a number of Secret cards that can trigger this easily (Wormhole from the very same set!). It is significantly stronger than Dragon Soul, and should at least cost more.

Seriona is also a truly scary card. Filling your hand with 0-cost dragons, some of which have extremely powerful battlecries, seems way too good. 

Goldrinn - This is a personal preference, but I don't like cards that swap stats multiple times. Its not an easy to understand effect for new players. Goldrinn is a great lore choice for your class, but otherwise I don't get the flavor.

Witchwood Guide - Do you have other cards that give Scutiny? Otherwise it might be good for the Guide to say "At the start of your turn, add a spell to your hand that Gazes a character."

Dungeon Delver - Possibly OP. However the class lacks 1 drops, so here's one. Might be better to just draw the Secret.
Lesser Obsidian Spellstone - Big defensive boost but you need to use weapons. 

Foreman's Pickaxe - Simple token generating weapon. Compare Piranha Launcher.
Refurbish - Weapon Tinker will want to quickly cycle through many weapons. This is a cheap way to do so. All Tinker weapons are balanced around 4 Mana.
Random Encounter - DND themed card. RNG minion creation, but you can stack the board in your favor.

Sprocket Thief - More weapon cycling.
Crushing Gears - Execute on a Secret. Probably the class's strongest Secret.
Clockwork Dragon - Recast your Secrets for big defensive value.

Titanstrike - The heart of the Tinker Weapon deck. Cycle through weapons, then play this. Other weapons increase attack, deal random damage, summon tokens, and add Scrapbots to hand. All together, with a Basic card that gives your hero Windfury, is how you end the game. In WoW, this is a gun forged by the Keeper Mimiron. The very same from the Mimiron's Head vehicle which isn't his actual head.
Gadgetmaster Gruu - The first version turned cards into Secrets, but that was too similar to Clockwork Dragon. This new one is a big Mech summon to, again, close out a game. There's a Witchwood card that helps this as well.

Mischievous Spook - Need another 1-drop, and here's a much more interesting version of Gnomeferatu. My hope is that the swapped cards wouldn't be revealed. Seems like something a mischievous spirit would do. 
Ominous Fog Machine - Sometimes you don't want your opponent to pop your Secrets right away. They might be better later on. This reverse-Flare is the key!
Gaslight Sentry - Secret synergy and added defense. 
Auto-Loader 3000 - More Secret refill. In a Secret. Secretception.

Protection Racket - Sure, we'll keep your town safe. Don't mind all the Mechs. Works well with Gadgetmaster Gruu.
Obsessive Inventor - This town sure attracts a lot of odd folk. Spell generation is a minor theme that was shown with prior set's transform cards and will show up more in the future.
Wrist Rocket - Double Savagery in a class that uses weapons. Brilliant!
Rusty Simulacrum - Reverse Muckmorpher. The resulting minion is a Mech instead of it's normal tribe (or in addition? can we do that?)

Screwvolt, Mad Inventor - Huge card for the weapon deck, helps you cycle through weapons, although there are a few that won't buff Titanstrike (including, ironically, the Titanstrike itself).
The Scarecrow - Play your cards right, and you can bring this creeper back over and over, wearing your opponent down over many turns. It's like a horror movie monster. But the initial cost is pretty big.

Lesser Obsidian Spellstone - More survivability  provided you use your weapons. Upgrades 4 > 8 > 12
Scrap Finder - This should be Rare. Quick Scrapbot generator that also triggers the Un'Goro Quest.
Refurbish - Cycle quickly through weapons to buff the Legendary weapon. All Tinker weapons are balanced at 4 or less Cost.
Crushing Gears - Execute on a Secret, sorta.
Clockwork Dragon - More Secrets!
Gadgetmaster Gruu - *deep breath* MOAR SECRETS!
Finkle's Multitool - Requesting better art if anyone finds it. All your weapons at once. An excellent finisher after playing a bunch of others.

Clockwork Hound - Too good for a 1-drop? Probably.
Gaslight Sentry - More survival tools.
Auto-Loader 3000 - Changed one of my original ideas. I considered letting it give you 3 Secrets but thought that was too good.
Wrist Rocket - Not sure of the balance for this.
The Scarecrow - A persistent threat. Play it, let it die. Next turn, spend all your mana and resummon it. Keep chaining cards right and you keep bringing back the creepy cursed Scarecrow.