For 6th edition, my wishlist items.
Wishlist #1 - Base cover for area terrain to 5+. But, use MORE cover than the 25% eyeball test from 5th ed.
Wishlist #2 - Improve Rapid Fire shooting to full range. Or make some sort of AOE suppression fire rule.
Wishlist #3 - Add pre-measuring. Seriously, modern weapons should have range finders.
Other ideas from alternate games.
Here are some Pros/Cons from other game systems that I am familiar with. I don't think all of them would work in 40k, but for consideration purpases it could be interesting.
Warhammer Fantasy 8th edition
Pros: Stand and shoot can be deadly, higher strength weapons reduce armor saves, pre-measuring
Cons: Ranged penalty modifiers stack up quickly
Pros: Shooter and target movement modifiers, very straightforward cover and LOS mechanic
Cons: Tokens or booking-keeping required to keep track of modifiers
Pros: Aiming bonus for standing and shooting, cover saves determined from base sizes, not actual model (no modeling for advantage), rules for multiple shots (RoF), all unit/models can split fire
Cons: Very short ranged weapons, stealth is generally over-powered
Pros: Overwatch (2nd edition, Space Hulk), wounded models suffer penalties to shooting
Cons: Wounding/incapacitated rules hard to translate to larger scale
Pros: Fluff actually represented in rules, bolters are awesome
Cons: Shooting mechanics feel slow and clunky
Here's hoping for the best edition so far for Warhammer 40k. Oh, and a 6th edition Tau Codex....