Given all the assets they spent time developing for the game, just making them available to the player would be easy and doable. That would be the first thing to do.
Like, there are tons of awesome animations played during main or side stories. Would be easy to use some of them for more quests and for romances. Would be nice to have more things to do with your romance option after you unlocked them (and they already improved on the Witcher 3 with the texting mechanics). Would also be nice to use the animations made and already in the game for when you're eating and sitting and make them available so you can sit and have lunch even when outside of quests.
There are also tons of outfits on NPCS which aren't available to V yet. They need to fix that.
More than outfits though, they need to make options available for your V to customize their own implants. I've seen golden arms and heads, neon lighted legs, awesome designs on NPCS. Would be cool to have the possibility to put implants on you not because of the gameplay bonuses it would bring, but because of how cool it looks.
Of course they need to add transmog, given the quality of the designs in the game. That would be the only way to do it justice.
After that, a bit of tweaking here and there and a bit more interactivity overall. But all the assets are already there. The animations are top notch, the designs are putting other games to shame, and the scenery / city is done. All in all, I'd rather have a strong basis such as we have now, with some things lacking and a bit of work needed to make the world more interactive, than the other way around.
Then, when it is done, more story of course. Those characters are so awesome and memorable, it would be a shame not to give them even more quests. Judy, Panam and the family, the Peralez, River, Claire, all the gang leaders, the fixers... so many wonderful stories awaiting to be told in this setting. This game is just begging to expand. Its potential is the highest I've ever seen.