The easy "fluffy" wish: "Personalized" subarmies in the various "generic" codices. Pretty much giving bonuses to different chapters, legions, craftworlds, etc. Crimson Fists could get Preferred Enemy vs. Orks. Iron Hands get the Bionics special ability at least on anything above a plain Battle Brother, etc. Deathworlder IG get +1S or T or I to simulate whatever has been selected for in the population. Various forms of elite trained IG could get +1Ld. This would quickly get into the "hard to balance" area because for some codices it could run into combinations of units and special abilities that could be lethal on the table. Otherwise, IMO, most of these could be simulated by either a flat or percentage cost for one Vet skill and/or Stat upgrade for ~3-5 points per model.
- Add a Movement stat to the units. Say 3" (Lumbering slow stuff), 4" (Normal human stuff), 6" (Fast stuff). Which means some MC & most Walkers would be 3"; IG, most MC & some Walkers - 4"; a few MC, Power armored or extra fast models -6". Jump packs/Flight & beasts/cavalry would give double movement. Assault movement would be unaffected, both for balance and because it simulates a "burst of speed" for the slower things. The purpose is mainly to provide a better reason for transports (assuming they take a hit in 6th Edition) and to turn power armor into something besides a fancy set of carapace armor.
- Range, weapon type and movement modifiers to shooting. The idea is to adjust the BS skill by the mods and use the current system. Shooting unit moves: -1 BS unless Assault weapon, Firing a Heavy Weapon while moving: -2BS, Target unit moves 6"+ : -1BS, Target unit moves 12"+: -2 BS, Shooting unit doesn't move: +1 BS, Sniper weapon not moving: +2BS, Fire with half weapons (Bracing weapons/Aiming, rounded down): +1BS, for each 12" range bracket after the first: -1BS (IOW, no mod up to 12", -1BS for 12"+ to 24", -2 for 24"+ to 36"), etc. The problem is that it would require a chart of modifiers to be learned, complicating the game. Also, some adjusting of BS would be required. Although some of it can be balanced trivially, for example, if the idea is that all base Marines can move normally, fire at up to 12" range at a target with a 2/3 chance to hit, then you'd make them BS5, a +1 gain to offset the movement penalty. However if they stand still with RF Bolters to shoot one shot at something between 12-24" away, their effective BS would 4.
- Overwatch/Final Protective Fire: I don't think I'd want a full "overwatch" rule, say roll Ld, get to shoot the enemy just as they charge into assault, no "Blast" weapons and "Templates" auto hit four enemy models but only with weapons that were not fired in the unit's last shooting phase. Possibly too complicated to keep track of. Oh, the charging unit's hip shooting as it comes in doesn't count, that is what their previous shooting phase was about.
- Armor Saves vs shooting adjusted by AP. This does not touch invulnerable or cover saves and models with only invulnerable saves do NOT get any modifiers from this. This does change up some of the worth of cover and invulnerable saves some, but I think produces a more "fluffy" result. Pretty much, every model gets a 5+ armor save vs. the same AP, then adjusts it by +1 for every level their armor save is better than the AP and -1 for every level it is worse. Ahhh, this is easier to see in a table and I cannot seem to get a table in Weebly. Vertical axis is "-", 6, 5, 4, 3, 2, 1 for AP. Horizontal is "-", 6+, 5+, 4+, 3+, 2+ for armor saves. Fill the diagonal from top left to bottom right with "5+", then as you go up each column the progression is 4+, 3+, 2+. Going down the column, it is 6+, "-" (No armor save). What does this do? It means heavier armored models shrug off lighter weapons easier, but are more vulnerable to weapons with lower AP.. Power Armor is 3+ vs Bolter fire and 2+ vs anything lesser, but are only 4+ vs AP4, 5+ vs AP3 and gain 6+ vs AP2. Yep, Space Marines will be wading through lasgun fire a lot easier, bolter fire just the same, but will have higher casualties from heavy bolters, autocannon and will actually have a slight chance of surviving AP2 fire. The only way to get autowounds will be to have one level greater AP than AS. Will this idea mess with everybody's mathhammer!
- Vehicles with sponson weapons should get +1AV (max AV14) on their flanks and "Fast" if they don't take the sponsons. I'm sorry, sponson weapons are just too WW1ish and this also allows for builds that field Predators sans sponsons as actual tanks that move and shoot in a mechanized assault rather than sit & hide trying to cover their flanks from shooting. The same would work for Leman Russ tanks.