A frequent criticism of the "seekers" in poly is that they are treating their prospective partner(s) as something of a toy, a robot, or an extension of their fantasy of what poly life is, without really trying to deal with a real person and accept the fundamental change which will be brought about by having a relationship with someone.
I think in part this is due to the fact that people are complex and individual. It is difficult to imagine a whole person, their dreams, fears, experience, spirituality, thought processes etc.
For example, in a sexual fantasy I would imagine that very few people,if anyone, gives the "other(s)" in their fantasies a complete life, history and personality. The "other" is perceived in one aspect only, serving to enhance and help fulfill the experience of the fantasist.
I think this carries over into the seeking, especially if new to poly. The "potential partner" is fantasised/fictionalised creating a silhouette into which a real person is fitted.
For someone who has had experience of poly it can also be hard. They, through experience, may have some ideas regarding possible issues which could arise, but still, if they do meet someone, that person is an individual and so will be unique, and must not be fitted into the silhouette of a previous relationship. There is also the point that a desire to "regain the bliss" must not turn into obsession.
I think it is important to be "open to the possibilities" rather than trying to imagine what poly or a new poly relationship will be like and to accept that everyone is so wildly individual that the changes they will bring will be individual as well.