1. Develop something.
Very few developers actually develop anything. They take old, terrible mechanics and rehash them as the new focal point of the game. Stop this. Only do it if the mechanic is actually worth pursuing, such as the FFT class system. However, if you are going to pursue something like this, DON'T RUIN IT LIKE YOU DID WITH FFT:A. >=(
2. WW2. Stop it now. I don't care about new locations. I don't give a shit about bombing Japan. I'm tired of using...
3. Guns. Can we get something else here? Anything? Star Wars has shown us that it is absolutely acceptable, and popular, to not have things revolve around hand held weaponry. Swords, magic, etc. Combat is almost always more fun when you beat the crap out of someone with a weapon. Note: Gears of War chainsaws. Note: Crowbars from Half-Life. Note: Swords from... no, the swords in Halo 2 are stupid.
4. Make more games like:
A. Bioshock. Extend the game by adding more to the story either through more characters you meet on your path or with just making it longer. Add multiplayer COOP akin to System Shock 2.
B. Portal. Again, extend the game by adding more to it. Add special COOP missions where it will require a certain number of people to complete objectives.
C. Oblivion. Take out terrible skill system, replace it with a system similar to UO's and give it up to 8 player multiplayer, 4 player coop, and 16 man arenas.
D. Might and Magic. Take the combat, put it into Oblivion. Add COOP. Simple enough.
Mainly, just put COOP in games that should have COOP. Go the D2 route and increase the difficulty of mobs per player in the server if the game is COOP.
5. There are certain formulas that work and should be reused and reused a lot. The first is the D2 formula. The D2 formula consists of simply COOP slaughtering of tons of enemies at a personal, and markable, gain either through loot, skills, quests, or otherwise. Many games have half-assed this formula and have failed. Some have gotten away with it, but those games usually fail in the replayability department (Oblivion, as an example).
6. Stop copying WoW. WoW is popular because it combines a ton of mediocre content into a colorful package. Stop trying to emulate this. Please. Just stop. You can't take players from WoW using WoW. Stop. Now.
7. More bright colors. Use dark colors when the atmosphere is right. Also...
8. Stop using semi-futuristic apocalyptic themes for your games. It doesn't make them edgy or cool. Only Fallout can pull this off.
9. More medieval FPS, less medieval RPG.
10. For the love of everything holy in gaming, redo the FFT class system in a new game with different classes. No, not like FFT:A did it. Just do a copy+paste of the general idea and then put in your own classes and skills. Please.