Perhaps the plot ideas could come from thwarting potential Yeerk infiltration plans
:
Idea 1: The Yeerks install and entrance to the Yeerk pool at a maximum security prision.
Idea 2: A new batch of Yeerks is dumped on V3 and he must find them hosts rapidly. To this end, he is given relatively more freedom regarding this, but if he's successful he could convince The Council of Thirteen to approve his all-out war dreams. He reacts accordingly: The Yeerks gridlock downtown and use a scare military campaign in an operation to rack up hundreds of adult specimen.
Idea 3: The Sharing has designs on a senior home? Cassie's grandmother is there?
Idea 4: A recycling plant, run by members of The Sharing, secretly concealing a Yeerk Pool entrance?
Idea 5: A hipster concert of some sort? A successful hypnotist Controller of low morals, designs a plan to gather a lot of susceptible young people that will get co-herced into accepting a Yeerk slug. The Sharing might even oppose him, for good measure and discombobulate the 'Andalite Bandits'.