To answer your last question first, yes, it's at-will, no-rest shifting, paid for by being limited in choice and by gold costs to get new forms. I'm a little unsure if I've overpriced the forms; I could see as little as 100x rather than 1000x, but I figured it's better to err on the side of caution, here.
An Invocation for Fiends (and other Types) might be a good one. Dragons might be fun as a sort-of capstone-level thing. 18th or even 20th level requirement? I hesitate to make it too "clean" to get gifts from other creatures, because there is a hidden cost in real play to having to actually kill something: you lose the potential ally.
The choice of Monstrosities was, however, based on the fact that they are, essentially, "magical Beasts," so seem the most natural upgrade from "only Beasts." While some are fully intelligent, they retain a quality of "animalistic" to them that keeps the Skinwalker vibe.
I almost made one that let you skin humanoids to assume their forms as disguises, using a more "keep your own stats, but you look like them and maybe get their languages and/or one of their proficiencies" approach, but I felt that it was a little too dark for something that other Invocations can already do most of. The
disguise self and
alter self Invocations handle looking like others much better, and still "fit" a theme with this Pact without being locked to it nor required choices. I feel like picking your type goes into similar territory.
Maybe something in Acolyte of the Skin that gives a "perk" for each kind of Patron. Fiendlocks can skin fiends. Feylocks can do so with Fey, and GoOlocks with Aberrations. Not sure what to do for Celestial Patron and (ugh) Hexblades.
Or maybe it should be a separate invocation that requires Acolyte of the Skin and a particular Patron for each kind:
- "Fleshwarper" for GoOlocks, lets them add Aberrations to their options and...something about grafts, I think. Not sure yet.
- "Faerie Bargains" for Feylocks, lets them add Fey and get their forms from gifts and bargains with creatures instead of skinning them.
- "Inner Demon" for Fiendlocks (alternate name as "Inner Devil" or something for different Patrons), which adds Fiends to their list, but also lets them pick one kind of Fiend that they could hunt and have that become their "true" form. They shed their own skin, which becomes something they can don or doff, and which is replaced like a fiend's skin would be if damaged to the point they can't use it.
- Not sure I like this one; the theme is nice, but the power seems too high.
- "Last Rites" for Celestial-Patron Warlocks: they add Celestials to their list, and don't have to participate in the killing of the creature. If they find the corpse of one that died within one week per Warlock level, they can perform the ritual over the corpse, and most of the corpse will vanish to the Celestial Planes while an appropriate token is left behind. (Costs are as normal.) Celestials may voluntarily stand for the ritual and be sent back to their home plane, otherwise unharmed, leaving their token behind.
- I can't think of anything good for Hexblades. I blame their unfocused, slapped-together fluff that makes no sense.