Internal vs external motives
My watch collection consists of whatever I want. For some that may focus on a brand, a type of watch, recognizable icons. Personally, I want a limited number of pieces that I thoroughly enjoy. Excess pieces just to fill out some sense of scope or diversity would detract from my enjoyment of the pieces I truly like the most.
If a collection must represent, in some small way, the universe of watches, then external factors will determine your collection. That is ok too. Many will find great satisfaction in having a watch from a certain range of brands or a diverse set of watches. Common approaches to this include things like the dress watch, chronograph, & dive watch collection. Another approach is a focus on Rolex & Omega. Others may stick with the traditional AP, PP, & VC.
I stick to what I like and enjoy. My appreciation of many pieces occurs via the internet and books, rather than by wearing a watch on my wrist.
The focus on assembling a collection of pieces that resemble the opportunity set of watches that are out there is an externally focused approach. In this method, if a new brand or type of watch is added to the set, then your own collection is rendered incomplete. If you focus on what is popular, then you will always be chasing trends, maybe even to the exclusion of what you really like. That is ok too if you find satisfaction from your collection.
My own focus is on deep appreciation of specific pieces and long term ownership. I don't really trade or sell pieces. Therefore I am very careful when adding something and then I enjoy it for the long term. I like this approach as it fits with my personality.