This is just me thinking out loud. These would all be six-month trip ideas.
1. The seven little countries south of Mexico and north of Columbia, followed by sidetrip to Cuba, then south to Ecuador, Peru, Bolivia, Chile, Paraguay, Uruguay and Argentina. (15)
2. Nine southernmost countries of Africa, plus Ghana, Morocco and Tunisia. (12)
3. India (mostly) plus some Nepal, Bhutan, Bangladesh and Sri Lanka. (5)
4. Georgia, Armenia, Azerbaijan, sidetrip to Erbil (Kurdish territory of northern Iraq), followed by eastward to Kazakhstan, Siberia (Russia) and Mongolia. (7)
The first three could happen any time of year; the fourth realistically only in summer.