The thing that I would love would be if the "Life" line was a place you could put your permanent stuff that would apply to all builds that got subheaded under it: Remnant tomes, past lives, universal/racial tomes, and even bonuses from favor rewards if you want to select those.
Then each of the individual builds would pull from that one datapool, and adjust accordingly. It would mean that when you are making builds, you only have to make changes to one line when you get another past life or upgrade your remnant tome, instead of having to go through all the individual builds and change all of them. When you make a brand new blank build for say, Dragon Disciple once that comes out in a couple of months, then your new blank build will have your past lives filled in already. If you are fiddling around with 20 different builds, and then you earn Completionist, it would be a big time saver to change one line and have it propogate the bonuses through all of your builds instead of having to go build by build (with all those load times) and change them all individually.
This would also free up screen real-estate space on the individual builds because none of those things would need to be listed.
This would also allow for some nice flexibility for people who were tracking builds for multiple alts (who all have different past life totals), or people making new player guides to get the different baselines separate and not have to individually track everything for each category of build they wanted to make.
Right now I'm kind of duplicating this effect by keeping Build #1 a basically blank build with no feats or skills selected, and I only make changes to the past lives etc, and then when I want to do a new build I make a copy of that one and make changes to the copy. It...works...but if I want a new past life to be reflected in the other builds I have made, I have to go one by one and make the adjustment to them. It's not a huge deal when you add say, one Bard past life on a Wizard build, but for things like EPLs that might be changing your count of spendable destiny points, it makes a difference.
Obviously that's a pretty big overhaul of the builder.