The Serenity comics seem pretty plausible for filler episodes and I think Book’s story would have been spaced out over a few seasons.
The Buffy comics were vastly different and Joss has said so himself.
I think the Dollhouse comics probably would have fitted nicely into an Epitaph episode or two.
The Angel comics might have been quite similar, but honestly I wish the series had ended where it did in Not Fade Away. It was perfect.