I’m currently working on revising Together is Enough. And as much as you try to fix problems during revision, you also have to work from areas of strength.
The middle of the manuscript is a mess. The hero doesn’t have a subplot (or flaws, really) and much of the middle of the book exists because I wasn’t ready to get the events in the last third. But rather than fixing lackluster scenes or expanding on minor characters that I don’t really like, the solution may be to take the things that I like about the first third of the book and expand on them.
Build more tension and drama into the first part of the book. Take it and turn it, in other words, into the first two-thirds rather than trying to fix what’s probably unfixable.