Topical Stamp collecting is a term used to describe a stamp collection of a single Topic.
Also know as a Thematic Stamp Collection
For example you could collect Stamps with the Topic of animals like the Stamps below from Transkei
You could also include First Day Cover Stamps with an animal Theme
Maybe add a few Stamp Presentation Packs or Post Cards
Your stamp collection could be more specialised by only collecting say...
Farm Animal Stamps,
Woodland Animal Stamps,
Domestic Animal Stamps...
Zoo Animal Stamps like the Stamp set below from Guernsey
This would make your Stamp collection range a bit easier to manage,
Still to big a collecting theme?
Maybe start with just one animal,
Dog Theme Stamps like the GB Stamps below
Topical Stamp Collecting is only limited by what you decide to collect,
I am sure with the many Thematic Stamp sites to browse you will find an area of interest to you!