The only reason I stopped liking Rugrats (not a lot, but a bit) was the spin-off. I enjoyed All Grown Up, and the series finale for Rugrats was great, but All Grown Up just wasn't as good. No scary adventures fighting under the bed or anything
I for one liked Kimi and Dil (but since he shares the same name as me, I don't like him that much
)
SpongeBob won't have a new character. Even Nick and the new writers know that would crush SpongeBob. It may be for the best, but it'll still get rid of lot's of fans. However, I'm not one of those fans who will just give up when a new addition is introduced.