A sequel to Square Enix's R.A.D. Giant robots fighting in huge cities, raining destruction down on everything around. Maybe make it so that your actual character can really fight this time around, like maybe give you a choice of a power source, like psychic fighter, cyborg stuff, magic, etc. Let you set your robot on autopilot so you can run into a building to save people or something while you get to catch glimpses of your chosen giant robot battling out of the windows you pass by.
As far as original stuff, well, I'd love to see a Grand Theft Auto-ish zombie game that really focused on survival. Like you need to find food and such to just survive day to day, along with all the normal weapons and ammo, as well as building supplies to fortify whatever place you're holed up in. Make it something like, well, maybe the zombies aren't so active during the day, some of them are out, but most are forced into dark places and such. There's other survivors for you to gather up and each will add to your defense and your ability to gather things, but they also place a strain on your resources, and then you have some special survivors that you can find that really add stuff to your base and whatnot, kinda suikoden style, some can craft certain things for you, a chef can make your food go longer, stuff like that. You can either go the solo route, or start gathering up people, but more people means more zombies coming at you at night. Also a few different 'bases' spread about the city, so as soon as you clear one out, you get to face a choice of splitting your people up between your holdings or solidifying everybody in one big place.
Things are complicated by the fact that there are other survivor groups out there to compete against, and while you could perhaps take them on on your own, you really need some people at your back to pull off pushing them out of their own bases. You raid them for supplies, they raid you. You can maybe even try to reach a truce with them, but most of them hate one of more of the others. Maybe have a group of former criminals and gangsters, group of semi-rogue military guys who got left behind in the quarantine, the corporate scientists and their mercenary buddies, and maybe even a group of former law enforcement and emergency service guys. You have to choose between staying in the city and trying to chance it in the countryside.
Combat would be varied, you could go at it like Sam Fisher, stealthing about, or maybe just try to be like Master Chief, and go in guns blazing.
Real fun comes in with how the game will change depending on how many recruits you have. Get one guy, you've got a buddy tagging along with you like Army of Two, get a few more, suddenly you're playing a squadbased shooter like Rainbow Six. Keep getting survivors together, and suddenly it's like an RTS where you have to coordinate the missions of several squads, to either retrieve resources, guard areas, take out bad guys, or much more. Keep going, and it's almost like an odd version of Civilization, where you have to manage your entire group of survivors, dictating which areas of tech they look into, like specially crafted weapons for your forces and whatnot.
Toss in some rpg elements, like you level up and you can focus on stealth, guns, crafting, and whatnot. Stealth skills are all about you getting around without being seen, maybe even toss in some parkor type skills to help you leap around the city, guns would help you aim better and maybe increase your damage with certain weapons or give you some special abilities. Crafting is all about you making better gear and improving the things you already have.
Sounds fun to me anyway, but it'll probably never happen.