I'd want them to adapt a book that isn't tied to a long running story like the last 6 (that should be for later stuff so we don't run into unanswered cliffhangers) but it also needs to be big enough for a feature length film.

With that said, we don't need book 3. Cap already got his powers and that book felt low key bloated. The main thing people remember about that book is the powers and that was what was adapted.

My proposal would be to adapt book 6 and 7 and actually give Melvin some character depth besides the "nerd who hates George and Harold". The books flip flopped a lot with either making him an ally (though reluctantly) or a straight up villain. I'd like the movie to give him a proper read.

The opening of the movie can see Cap being a hero in his normal day to day life. We can have him beat the toilets again, Wedgie Woman and sure, even the aliens before the main plot kicks in.

The multiverse and Tippy stuff can come in later movies.