Independently of playtesting the Slayer, the class I think has a lot of fun elements that I think are worth either including or developing beyond just them:
Full disclosure: I'm a big Buffy fan, so when the name "slayer" comes up, she's what comes to mind first. "Monster of the Week" is a format used for both shows (like Buffy the Vampire Slayer!) and games (it's even the name of a tabletop system purpose-built for it), and I'd argue it's both quite popular and quite good for tables wanting to play monster hunters of any kind, such as a Witcher-themed campaign. With some helpful guidelines on how to set up and run these adventures, maybe a subsystem to help, this kind of adventure format would be even easier to run in Pathfinder. As an added bonus, it would suit the Slayer's mechanics perfectly!
I love the core idea behind the Slayer's trophies. So much so, that I think it ought to be developed much further as its own system: there's been a lot of demand over the years for a monster parts system, and third-party developers like Battlezoo have famously made excellent such systems. There are, in my opinion, a great deal many ways in which characters could make use of that kind of system, whether it's your monster hunters reinforcing their gear, some kind of blue mage build using monster trophies to emulate the monsters' abilities, or even a Dungeon Meshi-style feat to make delicious meals out of various monsters for lasting bonuses.
To this day, the Ranger still only has the three hunter's edges they released with. This makes me a bit sad, as the class could stand to have many more subclasses that let them hunt their prey in different ways. In fact, an edge that quickened the Ranger for actions against their prey in a manner similar to On the Hunt I think could work very well for the class.
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All of which is to say: the Slayer I think brings a lot of cool ideas to the table that haven't been all that developed yet in Pathfinder. Although some of these ideas ought to stay exclusive to the class, many I think have applications beyond the Slayer, such that it'd be helpful to have broader support for adventures where the whole party's hunting a new monster every session or so and collecting trophies for various purposes.