Great story so far; interesting back story that fits into the world well. Sad that North Blue wasn't explored as much, but it is interesting to see the ideas for islands.

It is a fantastic concept that could have been accomplished better. I love the idea of OP MC Grandfather character that fixes the multiverse; it just needed to be done better. He doesn't act like a kind grandfather with wisdom but more like an arrogant person who thinks he can do no wrong. The story also needs to catch up more in a fix for each world rather than a systematic fix. For example, in the Harry Potter world, he fixes characters' lives years after he should have learned about them. It is readable, but it is easy to skim through everything and notice what happened based on a quick read-through.

Who?

You're writing too much of the original scenes. As a fanfic fan, I think most of us recognize what originally happened, so a quicker snippet of it could work to avoid leaving out the MC too much. This read too much like reading the actual episode of Castle rather than your own writing.

Thanks for the chapter. I would love to see it keep to Gossip Girl and Castle as the main, with Suit on the side, and some slight crossovers with others (nothing too deep; something like the Psych chapters is perfect where they intersect a bit but don't really get into one another).