OP animation definitely improved as it progressed into new arcs. I don't know, but I liked and missed the old art style (the ones where most of them were tanned).

They're really goofy there, and it's more like they're more serious when they return to Sabaody (well, who wouldn't after experiencing that traumatizing separation...). Nevertheless, I really agree with you that this OP Fan Letter is prolly one of the best filler episodes because it's really refreshing to delve into OP world from different POVs.