Here is an article from GamesRadar, Nine Ways to Improve Call of Duty:
http://www.gamesradar.com/f/9-ways-to-improve-cod-modern-warfare/a-20090114101841281049
I posted a few snippits from their more realistic and more interesting ideas.
Not Another Fictionalised News Story
[...]We don't want a re-run with a different villain, and a made-up World War III just wouldn't have the resonance that Infinity Ward CoD's are legendary for. [...] Our current favourite idea is to take things right out of the traditional armed forces and drop us into the pay-per-bullet boots of a private military contractor. Given the use of these just-about-legitimised mercenaries by world governments in conflicts all over the globe, there's the potential for a seriously varied campaign, especially as we wouldn't be tied to any particular nation or regime. [...]
Weather
[...]Try staying hidden when a sudden downpour has you leaving trackable footprints in the mud. Or sniping when your quarry disappears into the fog. Or even having to rethink your grenade throws as the wind kicks up. [...] Every weather condition would bring with it a host of offensive and defensive bonuses and traumas, and if implemented with enough thought for balance could add as much to the game as perks did to CoD 4, essentially doubling or tripling the number of multiplater maps available. [...]
More Stealth in Campaign and Multiplayer
Ask anyone what the standout points of CoD 4 are and young Price's stealth missions will always be high on the list. They came out of nowhere with a change of pace and a density of atmosphere that kicked us hard in the face and kept us hooked from the tense, pulse-pounding start to a breathless, adrenalin-soaked finish. [...] Whatever the reason, we want the same agonisingly tense and lonely atmosphere combined with dirtier, riskier, hands-around-the-throat stealth kills. [...]
Destructible Environments
We once had a few fears that destructible environments might make a shooter too easy, removing the necessity for accurate aim and tactics in favour of indescriminate bullet spam. [...] We don't want any Bad Company-style silliness where we can blast apart anything the enemy could be hiding behind, but we do want to be able to put pressure on them by chipping away at the edges of their hidey-places. And if they're doing the same to us, then Modern Warfare will be more intense than ever before.
Water Cover
It might be the simplest of simple pleasures, but one of our favourite things to do in (Far Cry 2) is to drop below the surface of a river just before reaching an enemy camp, swim along submerged, and then pop up from our splashy camouflage to unleash soggy hell. We don't necessarily want swimming in the next CoD, but the ability to at least submerge up to our necks for a stealth approach would make us very happy indeed.