I think we all know what people are focusing on here. The collecting and use of trophies is clearly the main feature of the Slayer. And I think a lot of us are dissatisfied with the current options on what we can do with them.
The blade and mail signature tools are solid, but the other options are lacking. Blade and mail also cover two of the obvious things to do with trophies when reinforcing gear - weapons and armor. They're probably fine as is, at least the initial benefits (some of the upgrade feats are iffy).
The other two signature tools however are lackluster. Chemyst comes closer, but Ignition Vials is weak. Panoply is just bad. Both of these are trying to be hunting tools, which are the third category of things to reinforce and probably the broadest.
For Chymist's Vials, I'd drop Chymist's Eye entirely. Just focus this tool on Ignition Vials instead. The frequency needs to be more often, this should be available as part of our combat routine rather than once per fight. No, the level 7 benefit doesn't count, that's too late. For this, I'd make it 1d6 to start, +1d6 per two levels (slightly better damage scaling long term, and d6 is better to roll lots of imo as a practical question). Cooldown either 1 round or 1d4, depending. At level 7 you add 1 (1d4? 1d6 might be too much?) persistent damage of the same type used, increasing further at higher levels, as you've improved it to cause lingering effects. Because initial tools should have extra effects against your quarry, I'm leaning towards saying your quarry takes a -2 to saves against Vials as you adjust them specially for that target. Maybe also have the option to have it only damage your quarry (chosen when you use them). It's also possible this might be better as something you have X uses per day of, and can refill with 10 minutes to brew another, as pseudo focus points.
Chymist's Eye can be a feat instead, but allow it to steal any special senses from the monster slain. Maybe have a level minimum before you can take non-vision senses, but eventually being able to take things like Tremorsense would be worthwhile.
I'd add a feat to modify vials to allow for other AoE templates. Line AoE or 5' Burst within 30 feet, probably? You choose when preparing them, and the later feat to upgrade the AoE also increases all of these.
Panoply I don't really see any way to save. I like the idea of getting bonuses to saves based on the trophy, but that can be a feat (possibly linked to Warded Mail).
Instead of panoply, I think we should have an additional active tool or something available. While the vials are direct offense, the class currently lacks traps. A Big Game Trap of some kind would be a valid starting tool, where you can prepare them in the morning. The slaying technique here would be throwing the trap to deploy it mid-combat. The actual effect of the trap would probably still be damage, but also debuff (I'm going to say Clumsy 1 by default, but with upgrades and alternate debuffs available). Level 7 enhancement makes the traps larger (optionally), or maybe allows throwing two at once. Either way, more of the battlefield under control, more places you and allies can push enemies to hurt them. This is very much spitballing, but I'd focus this more on debuffs with the damage as a bonus, the intent being more to hit weakness than raw damage like vials. ...Maybe the default Clumsy debuff should get nastier at 7 too (or add Enfeebled or you can pick two debuffs at 7?)
As to feats not directly linked to gear - I think the Chemyst's Eye feat doesn't need to require any base tool. I'm assuming Slayers in general have some proficiency here to adjust their tools of any type.
Spirit Oil needs to be more reliable. An additional way to get damage is great, especially for non-Blade builds. But if it's one batch per day I think either it should always apply persistent damage or add some flat damage? ...Honestly I don't like the 1/day on it either, just making it 1 persistent per weapon die would work and allow it to auto-scale as well. If you do this you can drop the Deadly trait benefit probably (I like it, but...)
Cure-All is kind of bad. I don't know what to do here, but needing a full minute to use... it's really only relevant for petrification I think, because by then they can wretch away sickness and had better have been stabilized, etc. At the level you get it it doesn't do much too, it could be higher level and more potent of a patch (a mid-combat debuff cure is much more relevant too)
Adaptation Serums - the reinforced benefit feels too narrow here. I'd at least add half level resist to the damage type as a whole? Make it pick one damage type? I know this overlaps with Mail a bit but it's so narrow.
I'd definitely add a feat to allow access to alternate movement types. You can level gate what kinds of movement you can get with the serum, with swim/climb at any level and flight starting at 8-10 or something. Can be once/day, for sure. But melee builds love ways to ensure they can reach an enemy trying to keep away without needing an alternate weapon or anything.
