I could see a game in the same style as Genesis or SNES, ala Mega Man 9 and 10. However, an officially liscenced brand new game released on carts? Not gonna happen. Neither Nintendo nor Sega manufacture those consoles anymore, and have no real reason to manufacture games for it. As popular as retro gaming has become, the market is still niche. And with things like Virtual Console, Wiiware, and the like, there's no reason to make a physical copy of a game that plays in the same style. Hell, you can use the same style and not be restricted by the hardware, just like MM9 and 10. Those two games, similar as they are to NES games, are too advanced to be put into NES carts.
Now there are people who are making NES, SNES, and Genesis games now, but these are indie develpers who loved the old consoles and are trying to breathe new life into it for no other reason than nostalgia. For those games, find a kickstarter page and pledge, and hopefully they'll get made and you can enjoy new games on old systems. But they won't be officially liscensed.