i belive i got to this thread to late... but maybe with dlc/ expansion my suggestions will be considerd...
1) a deeper skill and gear customization, it was good in the witcher 3 but far from perfect. maybe add an additonal trait skills from gameplay choises a la witcher 2.
2) make every skill tree had an uber, turn tabler reacharable skill semilair to adrenaline from witcher 2, but more accesible (in witcher 2 you could had the skill the earliest in the end of act one)
that skill in semilar way of limt breaks and overdrive from final fantasy or devil trigger from devil may cry, can be tacticali be use agains powerful enemys and bosses.
3) make skills scale in new game plus. in witcher 3, new game plus was nice, but some builds were renderd useless beacuse they did not scale and do unntoicalbe damage to enemies.
4) one thing i loved in the witcher 3 game that there was a veriaty of end game gear depending on play and skills available. expand on that evan more and it will be evan more awesome.
5) maybe shield field generators like those in borderlands but make them eith a twist- like rare end game gear.
6)regarding story and charecters i really can't complain. keep up the level of writing in your previous games, and im good
7) like some said here already add custumable base where you can store your gear and personelize ahouse or apartment in the night city. a personl parking for your car and giant mecha will be nice
8) add vehicles customization. like spped boosters, improved handeling and stuff like that.
9) i know it's ment to be a humongous game but expansions are not nessceraly a bad thing if you do them right
and YOU can do it!
10) add optional super bosses, and make the final boss a really tough cookie. make them. make the super bosses worth the challange by making the rewards for defeating them epic.
that is what i can think of right now. i trust the game will be awesome!!!