Now in the official FAQ for the closed beta, they say “We don’t have anything concrete and confirmed at this time, but will update you all and let you know if this changes.”
Though just off of speculation, if we assume there will be some kind of small ingame memento for participating, what would you like to see? Is there anything you think would be neat?
An insignia, I suppose.
I would appreciate a skin for the VT’s owners
-get a rat pendant
-a skin for the hammer (sigmar’s hammer)
some cosmetics easter eggs would be fun
Something like a uniqely-colored small rosette cosmetic or a uniquely-colored purity seal would be a pretty cool commemorative beta item I think.
For instance (and just using this as a reference) in Team Fortress 2, players who were early players of the game, as well as beta testers, received gold and platinum-colored cosmetic badges, respectively. Something similar could be neat!
I want a weapon skin that looks like the head/face of a korn bloodletter, with glowing sight rails, like there swords.
Considering how vermintide 2 small rewards are, if they give us anything, will be a special frame/portrait for the character. They usually give these things for seasonal events. So don’t expect some amazing cosmetic.
Yeah, I wouldn’t expect anything really grand, just a small little thing. Fun to think about the possibilities though, right? 
A frame consisting of swarming Nurgle blight flies around our character portrait, since we survived being surrounded by bugs. 