Some random personal head canons of mine!
1. Modern Mandarine has the power to bend fate, but he cannot consciously control it nor is he aware of it. However, each time he was "reborn" other than the first time in season one and the hellfire situation, it's not actually just luck or prophecy, but him bending fate without realizing in a way that would benefit the greater good. Just so happens that he's one of the most important things for the greater good.
2. Fringo was only nice to the boys in season two for two reasons: one, he had no robot and he didn't want to anger them, two, he didn't like the ninja establishment. It felt off and he didn't trust it. Eventually, the boys became friends and it was no longer nice just for those two reasons and he was actually fond of them.
3. Modern Mandarine is given a priority status in the combination. He can kind of veto things, except the things he does can't be overridden.
4. Ancient Mandarine committed suicide and actually died soon after Pineapplello vanished/travelled to the human world.
5. Ancient Mandarine will almost certainly lose a fight against modern Mandarine. He doesn't have the strongest Sacred Sword, nor has he been reborn from hell fire, or had a genetic mutation. He's still lame.
6. Ancient Mandarine and ancient Applo are both stronger than the Pineapple. (Romance of the three Kingdoms, Bei is the leader but Yu and Fei are stronger?)
7. Pineapplello has tons of siblings named very similar names, one of which is “吹牛".
8. I think this may actually be canon, but Fringo and Pina are a better match than Applo and Blossomy.
9. Pineapplello is the second youngest of the boys, younger than Applo. Fringo is the youngest.
10. Three out of the four criminals are teenagers, while Bane is going into his thirties.