From my experience, the easiest for domestic flights is :
1. Japan (especially from and between HND, NRT, CTS, FUK, KIX, OKA, and ITM and also seems some of real-world routes). You can even fit a 24/7 flights (24 flights daily) all using wide body jets on some routes. Huge demand to neighbouring country such as Korea and China is a good bonus.
2. Indonesia especially from CGK, SUB, DPS + real world routes even from/to small cities has big, albeit not as big as Japan demands. Huge demands on route from CGK to SIN and KUL is a bonus.
3. China, lot of high demand domestic routes.
Other than that, never tried before. US maybe another one but competition is quite stiff there.
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