Orchids

Native Orchids of Australian & Singapore
Greece
Sydney Olympic Games ...

For example, if you choose flowers on stamps as a condensing theme, you can have a specific topic on orchids, and from there you can be more distinct with the types of orchids.
... preparing a checklist of themed stamps ...

" But "orchids" is a possibility. The trick is to find a topic that is small enough to be manageable, but large enough to supply a fairly regular flow of stamp issues.

South Korea issued two souvenir sheets (Scott 2057-58) picturing and smelling of roses in July 2001, and in November 2001, a set of four stamps picturing and smelling of orchids (Scott 2066a-d).

See also: Stamp, Pair, Philatelic, Used, Fish