It all depends what you want a fatbike for.
If you are going to use it around your trail centre just look for a fat version of your trail centre style bike (although plus sizes make more sense than fat for that IMO).
If you are going to use it for going into places where other bikes can’t go, or find it hard to go, eg snow or bogs or deep sand or icy ruts, then it’s all about the characteristics that make it handle well in those situations. I prefer the early models of fatbikes because they were built for that with little concession for other uses (eg steeper HAs because no need to provide for suspension). I have several fatbikes, and I still pick the early versions, ie Pugsley or 1st model 907 for what I call proper fatbike riding.
That said, first consideration on a fatbike should be the tyres and wheels. Tyres should be as supple as possible for ease of pedalling.