Hello shengami. The concept of taking on the role of a pair of Team Rocket grunts sounds interesting! If you are okay with me playing a female character, I would be happy to think up of something with you. In regards to your idea for the Pokemon League, as you need badges to compete, it may be an idea to play out obtaining them. Being Team Rocket ofcourse, they may not try to get some badges fair and square, instead trying to steal them, though they may think of battling to make sure they are tough enough.
And ofcourse, attempting to snag pokémon from gym leaders could be an interesting idea as they should be powerful pokémon. Though they may be tasked to just compete without causing such high risk plots, as that could give them away or make them wanted before they get to the league. Perhaps, while travelling about, they get to cooperate with colleagues on certain tasks set by Team Rocket, such as bringing an experimental pokémon hypnotiser part somewhere, securing a certain type of pokémon Team Rocket wishes to study or use (perhaps a physic pokémon for the hypnotiser mentioned before), they could try and beat competing teams away as they travel, sabotage certain places such as power plants or a training spot for growlites of the police, spy on certain trainers, try and recruit grunts, steal pokémon eggs, scare a flock of pidgeotto's away so fearows can harass people while Team Rocket gets resources or perhaps fossils from a certain spot, etc.
To show they are good enough, they may just be given one pokémon and they have to prove they are able to get a viable pokémon team going by obtaining them one way or another.
In the case of my character, perhaps an idea for her to rebel against her family is for her to join Team Rocket. Not desiring to stick with her family breeding the same boring pokémon, say, Mareep for the wool, she would rather go on adventure! She would despise how one family member, perhaps much older sibling or cousin or whatever, has been allowed to and is a succesful training in the Pokémon league, while she has been disallowed to. Having run away to join Team Rocket, she would love to teach said person a lesson.
What do you think?