Becoming demon is ultimately a matter of power. The demon king might or not have his own preferences but clearly the situation amounts to basically PL. Enough PL and obviously your power and it's.... depth increases (hence the bit about the demon king and supreme goddess being capable of even altering death). In that regard even the demon king can be thought of as merely being an insanely powerful demon. And that is why training to become demon king was a possibility for the meliodas and is also possible for zeldris and estarosa given the time.
With that in mind, since being demon king is a mere matter of power, I would argue a potential candidate for demon king can come from any demon. You just need the potential and talent to become that strong and hold that much power. It's hard to say why the demon king specifically wants meliodas to become demon king and not the "better" sons estarosa and zeldris but I don't think the manga rules out at least zeldris becoming as powerful as the demon king by training alone considering he is also a genius.
Another interesting bit here is that perhaps meliodas, estarosa's and zeldris' age are very telling regarding their talent. Meliodas reached 142k completely on his own... And estarosa is 380 years old as far as the series has revealed. Zeldris in turn is 252 years old and has higher pl than estarosa by at least a bit. Basically zeldris more or less caught up to estarosa even though estarosa had a 130 years head start. In turn estarosa is nowhere near meliodas. And the difference between them might not even be that big....
With that in mind, since being demon king is a mere matter of power, I would argue a potential candidate for demon king can come from any demon. You just need the potential and talent to become that strong and hold that much power. It's hard to say why the demon king specifically wants meliodas to become demon king and not the "better" sons estarosa and zeldris but I don't think the manga rules out at least zeldris becoming as powerful as the demon king by training alone considering he is also a genius.
Another interesting bit here is that perhaps meliodas, estarosa's and zeldris' age are very telling regarding their talent. Meliodas reached 142k completely on his own... And estarosa is 380 years old as far as the series has revealed. Zeldris in turn is 252 years old and has higher pl than estarosa by at least a bit. Basically zeldris more or less caught up to estarosa even though estarosa had a 130 years head start. In turn estarosa is nowhere near meliodas. And the difference between them might not even be that big....
