First things first... I'm happy to see an expansion for BW!
And while I somewhat dislike the sadly quite cartoonish look of most of the enemies (not the low poly graphics), it's really welcome to have a bit more of this nice game! Though I have to say... the expansion, just looked at on it's own, is a bit of a letdown for me. So I want to address some of the shortcommings as I felt them.

My main gripe with it simply is actually that it sadly doesn't "expand" anything.
So far it just substitutes 3 bigger maps with 1 sadly quite short one...
TBH, I would've like it a lot more, if the new protagonist would've been a NPC instead, that the original Avatar helped along. Most of the story could be the same that way. But the, as said, quite small map could then indeed be an addendum, a 4th map, instead of a single first.
Or maybe even in a hub like fashion a side-map, the Player can enter at will at or after the second Chapter of the main game, like some of the most classic games handled it. Like Diablo: lord of destruction, Dungeon Siege: legends of aranna, or Gothic: night of the raven.
That would also help with the amount of addition such an expansion offers, as additional content from the expansion could give new life to the original content, and thus increase it indirectely, as said games did nicely with new items/skills/mechanics etc. And for players that want to enter the expansion immediately, without playing earlier chapters, that's what a premade save/starter option takes care of very well and with minmal effort on both sides (player, devs).

I'd love to see something similar in a future expansion. Either as an expansion to the expansion or the base game.

Also a word on the new and expanded mechanics...
The new summoning options are IMHO a real nice touch! And go very well with the gritty n dark theme of the game. It's actually something I expected to see in the original already, at least calling those skull-spirits. Very fitting fot that oc.cult wild west setting. Having something like that in the expansion: thumbs up!
The way it is implemented though... less so. I only know of 2 summons so far, the wraith gunslinger and the spirit wolf.
First, both seem to have severe AI problems, as they often just idle around while being chewed up by enemies, making them almost completely useless.
Second, the ups and downs are way to extreme... even for the gunslinger. IF they attack... and IF the aren't killed in 2 or 3 strikes... then having a free bodyguard all the time feels quite a bit too generous for this type of game, while having just one-use per rest (making resummoning impossible) on the other hand, feels WAY to punishing if enemies chop them down with 2 or 3 hits, even with learned perks!
It would be better, IMHO, to even those out. More punishing ups, less punishing downs.

My suggestion for the "Head Pendant" for example:
- no limitation per rest or max HP reduction
- instead upfront health cost when equipped, like 20 or 30 HP or so
- and as long as the wraith is up a special effect active like "siphon life" that drains 1hp/sec for example.

This would make it less punishing if the friendly AI screwed up and got itself killed, as resummoning would be possible. But it would still be at a notable cost. It aswell would make health potions more useful even without being hit, as a sort of "mana potion". And it would have the effect of making the "easy going bodyguard" approach more taxing for each second, and way less "fire and forget". Exactly how I feel the grittyness in this game. A dark bargain the player should have an eye on all the time!
Which brings me to the 2nd tier of the "puppet master" perk... way to unrelated to the summon actually, let alone the fact that summons barely kill a thing in their mostly short lifetime, even less in the right moment for that stamina boost to matter... But... it instead could reduce the "life siphon" (or maybe other negative summon related debuffs) effect that I suggested earlier by half... Allowing to keep the wraith up for longer with proper equipment like strong regeneration artifacts. Even more dark bargaining, and all while avoiding the frustration of such situational effects like it has now.

As for the skeletal wraith wolves... it's actually hard to say, they almost never attack anything for me, so it's hard to say, but as they are melee n dying in 2-3 hits, and many enemies have strong aoe... well, so far I'd at least would make them usable from the hotbar aswell, for ease of use, and wait for an AI fix, then see from there.

In general... the "oc.cult/voodoo/magic" weapons feel a bit less tactical and too "spammy" here. I'd personally add a reasonable but not too low life cost to most of them to counter that and underline the "dark arts" aesthetics.

Finally, there 2 things I'm missing since the release of the orignal/main game... Both of which I think could be done with the introduction of an artifact. As already mentioned: a way to call those skull-spirits and... a lightsource.

- The lightsource maybe as a "cursed lantern", that illuminates the players surroundings, makes sneaking a lot harder for obvious reasons, and maybe releases a flash blinding enemies when aiming down the sights? Or a reusable item like the canteen? Something like that.
- The skull-spirit artifact maybe as a rare drop from vendigos? A "vendigo ribcage", that, whenever the wearer receives damage, a skull-spirit seeking out enemies is released. Would give a nice interaction with the spirit-bow and similar self damage effects, turning the passive effect into a controllable one. Maybe adding a 5 HP loss on top for every skull-spirit spawned. As a description something like that comes to my mind: "On close inspection not just one but countless little pain twisted skulls seem to be stuffed into this gruesome display of torment. You can feel their anguish... and it is clear to you, they feel yours aswell..."

Still... waiting for the next addendum, whatever it may be. :)

PS:
And... there was one especially ugly and unworthy low blow when playing the expansion I want to highlight specifically... the FORCED UNSUMMON when entering the final bosses arena!
If the mechanics make them useless (the wolves probably, the wraith should actually have some use) ... fine! The player will soon find out! But goodness I hope the devs stop defiling their otherwise great game with meta cr*p like that! The invisible walls are already bad enough...

edit...
Gosh it's not easy to get a post out here with that forum software...