In general, the spirits and humans should be alien enough to each other that distrust exists from that alone, and early interactions (possessions, attempts to bind to servitude) might have made things worse. Spirits being stranded on this plane and needing a guide sounds like a good motive, though. Bear in mind that the players can supply the motives for their characters, within reason.
One possible model I had in mind was the spirit plane was originally unstructured, but as spirits started falling through into the physical plane, structure started leeching into the spirit plane. So they'd come in as unfeeling globs of spiritiness, resembling meteors, then gradually acquire a distinct form and personality from interactions with the material world. Then sometimes a few of them would return, bringing distinctness and structure with them. This is just one possible model, though.
As for forms of spirits, they should be diverse: from flame-like/swirly energy things, to animal/insect-like, to really bizarre things with lots of wavy bits, and anything in between. Possibly demonic-looking ones too, but I'm a tad leery of that aesthetic here (I'm not going to veto it yet, though). But I'm not going to say "All spirits must follow X aesthetic," aside from the proviso that they seem insubstantial in some way. I want to let people be creative.
How alien is alien enough? My best idea was taking inspiration from invertebrate specialization and behavior, and attempting to assign a quasi-human personality to that. But you really can't count on people bringing something interestingly alien to the table if they're supposed to make a person in the first place. Xenofiction hardly ever works.
The unstructured spirit plane sounds rather amazingly boring at first, but I could see how you could do some fun things with it. Spirits imitating nature (and humans as well) without true understanding a la Dragon Age, and structure being introduced to the spirit world around the area of the passageway, with the edges of it forming a gradient of structured and wild environs. How about allowing humans into the spirit world, but the role of human and spirit is reversed there?
You could even have it be a question of awareness - the spirit world consists of three extra spatial dimensions, each orthogonal to every regular spatial dimension. Humans and spirits always occupy both worlds (they both have presences in all six dimensions), it's just a matter of learning to change one's perception from one to the other (neither a human nor a spirit can perceive six dimensions of space at once, so they can navigate either one or the other at a time - the alternation happening either in sleep, to further rip off Dragon Age, or as a learned technique that has only recently become apparent to spirits, who have then taught it to willing humans in return for guardianship in a strange new world). Maybe all regular life always existed in the spirit world, each the center of an island of structured reality? Each spirit, meanwhile, exists in regular space as well, and around them you get areas where strange things sometimes happen (before they started moving around, that is) - forests of suicides, a bridge that dogs jump off of, mysterious, illogical monuments occasionally found around larger populations, that sort of thing. Or maybe what we call plants and rocks in our world are spirits in the other one, while we humans are the plants and rocks of the spirits (this doesn't fit with the rest of the paragraph, but sounds cool in my head)?
And similarly to the world of regular life introducing order and structure to the spirit world, you could have the spirit world introduce unpredictability in the regular world. Entropy, so to speak. Just as you have humans probably wishing for a return to normality, you could also have conservationist spirits seeking the return of spiritkind to their native state of total chaos and unity, fighting the novelty of Earth life-induced permanency by any means necessary.
As for aesthetics, it's pointless to argue what aesthetics are appropriate while nothing at all is clear of the universe itself. Start from the fundamentals. Though that might just be my interest in constructing cosmologies talking.