I'd like to see greater starting hit points and healing surges, it was a shame they did away with them. Something I've done in my games, especially for level 1 starts is double starting hit points from class, so a fighter has 20 hit points + constitution modifier at level 1. I haven't used healing surges but I think a number equal to half your hit die size at level 1 + Constitution score would be good. This would give the fighter surges equal to 5 + con mod while a wizard would only have 3 + con mod.
I use the bloodied condition in my games to trigger effects and think it would be worth bringing into the base game.
I sort of use monster type as well, but normally that's just as a template to modify a monster, like soldier giving better AC, Brute more hit points but less accuracy, etc.
I wouldn't mind more weapon keywords to diversify weapons a bit. So many rogues just stick to rapier because it has the highest die size that can be used with sneak attack, would be nice if other weapons had a smaller die size but some sort of weapon keyword that would make the rogue player at least consider them.
Themes,originally from Dark Sun 4e, could also be a nice customisation option similar to the supernatural gifts from Theros so we already have something similar in the game.