Nice question. Here are things I consider when choosing a scale subject.
There has to be some personal appeal/interest in the aircraft. This is really an intangible thing that I cannot really explain. Either the airplane appeals to me or it does not. To do a really good job for a really good scale presentation, I really want to do this thing.
Generally, most models on my list are not often seen as a scale subject. This rules out most kits. (Besides, I do not generally like to build from kits because of wood selection, construction differences I prefer, control installation, engine/tank mounting arrangements and on and on.) Another problem with scale kits, is that some (most) have compromised some scale fidelity in the design. If I deviate from scale on my model, I will do it for my reasons, not for the ideas of somebody else. Another problem with kits is to find documentation that really matches what that kit represents, particularly if the kit does not have dimensions like span and length that are to the same scale, canopy shape and a whole host of other items where kits make some compromise from scale. (That may be OK for Sport and Fun scale, but not so good for Precision scale.)
Documentation. It helps to have some ready source of good 3-view drawings. This does not necessarily eliminate some obscure but otherwise "interesting" aircraft. With the internet, it is amazing how much material can be obtained (photographs, 3-views, books, color schemes, other material) in a short time. I have a list of several dozen airplanes that interest me for various scale projects including some for Profile scale, others for Sport scale, others only for Precision scale, some for the Carrier events, others for the 1/2A scale events that are showing up around the country, and others for semi scale stunt ships. I will pick up 3-views, books, photos, color reference, plastic kits, and decal sheets (which are great for color references). Bob's Aircraft Documentation is a great source of 3-views for aircraft that otherwise difficult to find. Persistence is key to this process of getting documentation and is part of the challenge of putting some obscure airplane together. (For instance, more than 30 years ago, I started collecting information on a one-of-a-kind aircraft built in England in 1944. I have found 16 different 3 views and have filled a file drawer of articles, photographs - many unpublished - and correspondence on this one aircraft.) Part of my interest in scale is to gather information and build models that are not often seen.
Needles to say, I will never finish my bucket list.
Keith