The information provided by the gunslinger TV series did not mention that the series had 40 episodes, nor did it mention that it could be watched for free. Therefore, I don't know if the gunslinger TV series has 40 episodes and if it's free to watch.
Snowball is a great character to include. He adds an extra layer of conflict as he also has his own ambitions for world domination. Another good one could be the lab scientists. Their presence can add some mystery and backstory to Pinky and the Brain's situation. And Elmyra, she can bring chaos as she always does in the original show, disrupting Pinky and the Brain's carefully laid plans.
Well, I think Mr. Itch could be an interesting character to include. He has his own quirks and could add some chaos to the fanfiction. The genetically - modified fleas could also be part of it. They might side with Pinky and the Brain or cause trouble for them. And how about the other lab animals? Like the hamsters or the guinea pigs. They could be either allies or obstacles for Pinky and the Brain in their quest for world domination. There's a lot of potential for different interactions and storylines with these characters.