Kits, Prestige class (and epic destinys) class archtype (I am thinking of it and kits like modern warlock 2 choices) psionics (as seprate) paragon paths, encounter abilities, complex martials, Advantage disadvantage, SoD/SoS effects (for everyone) level drain (mostly like 3e not 2e), leadership rules, paths to immortality (maybe through epic destiny) 20levels or more (I would love for each class to have 30+ levels with the DM having any point they can cap for the next one) Epic rules (so you can do Epic 6, Epic 10, or EPic 30 or anywhere between), Broad species choices, Theater of the mind combat (with OPTIONAL grid) Detailed travel/journey, Detailed social rules, skills, feats, 3 saves (or defenses or maybe 3 defenses and 2 saves...) prof bonus, xp from treasure combat and story, player facing item creation, templates for monsters, and random treasure tables
Over all I will add LESS hp then 3e/4e/5e but slightly more then 2e... and front loaded (like 4e) HD as the main healing mechanic (spend HD or heal as if you spent HD)
I have tried to design this like 100 times and failed, but basicly the unholy love child of 2e 4e and a bit of 5e is what I want...
my most resent attempt had me take artificer and warlock as my base classes refluff and rebuild them into fighter thief and magic user with a prof bonus equal to 1/3 level and a non prof bonus equal to 1/5 level and an 'expertise' of 1/2 level and 'mastery' as 1/2 level+1 everyone starts with 3HD and gets a HD at odd levels not even (where you get a set +1/+2/+3) you don't get con bonus to hp BUT you get con bonus to HD spent to heal...