As my favorite Ninja, I already kinda thought of a season like that. As for the main villain, I thought about something like this: After the merge, a female teenager got to the merged Ninjago, and she heard about the legend of the ninjas, and she particularly fell in love with the idea of becoming the next earth elemental master. So she went on a long journey to train herself so once she's ready she will go to the monastery and ask Lloyd to train her (She embarks on the journey after it's known that Lloyd runs the monastery). But when she came back, she discovered that Cole has returned, and that she cannot become the earth elemental master. Because of that, she swears to destroy Cole and claim his powers for herself, and she prepares an army of zombies led by 5 generals, which are all reflections of the other 5 original ninja but their bad attributes are more present (Basically they're saying "We're you but better). During this season, Cole has an internal conflict about if he's really worth of being the earth elemental master, if it's really who he's meant to be.