I've seen many games like Rayman Legends, Grand Thief Auto 5, Call of Duty Ghosts, and many other games released on next gen consoles (like the PS4 and Xbox One) into last gen Consoles (like the PS3 and Xbox 360) so in the perspective on other things, we might ask... What will happen if a Sonic Game looks on the last gen consoles as well as next gen?
This has been a tough question especially as the last gen is fading to next gen. By 2016, the Last Gen console years will end with the Xbox 360 ending support by 2016 and the PS3 ending it by 2015. Now that we're having games released on next gen, much of those games are starting to become a Multi-Generation Game, when games are released at two or more generations. This has happened to the first two Sonic Games on the Genesis/Mega Drive and Master System, along with SEGA Superstars Tennis and Sonic Unleashed (though different but similar to the Wii Version) on the PS2 and PS3 (which is different than it's PS2 counterpart). However, knowing if a Sonic Game could be going Multi-Generation is a major question. With some of my Research especially on non-Sonic Games, like the ones mentioned earlier, this may explain how this could be experienced.
There's not going to be a lot of changes to game play. The only differences to Gameplay is some minor controls, however there is some elements added. For an example, Rayman Legends added some touch controls on the PS4's DualShock Controller which is similar to the Wii U's gamepad. Towards that, the Wii and Wii U are really different though much of it's controls are compatible. The only problem is the use of the Wii U gamepad which is a question many developers are considering about. However, the Wii's Classic controller is discontinued as part of making their Pro Controllers so both the Gamepad along with that along with the Wiimotes are something they must worry about. Also, some games may need to change how some buttons are removed, Such as Sonic Generations with uses the Select Button on the PS3 and the Back Button on the XB360. The Back button is gone and it's replaced with a Menu or/and View Button for the Xbox One Controller while the Dualshock 3's Select Button is merged with the START Button into the Options Button for the PS4 DualShock 4 Controller. Some of these may need to be integrated to the new controls though this may not be the case to the Wii U.
In addition to that, the PS Move is fine so they're no need to worry about though they must get ready to know how to use it with the PlayStation Camera for the PS4 and the Kinect for the Xbox One has more different changes than it's original Kinect for the Xbox 360. Other than that, the Havoc Engine is Next-Gen Ready so there's no need for Sonic Team to worry about towards the gameplay engine though they must be familiar with the PlayStation 4 and Xbox One, especially with the similar and interchangeable hardware towards the PCs.
GraphicsThis is one of the biggest changes to the consoles as well. Towards hardware, the Xbox One and PS4 does have similar hardware with PCs though the Wii U is a bit limited. With better processors, GPUs, CPUs and others, it makes games more "Photo-Realistic" than ever before. However, Graphics on the next generation consoles looks very great than on a Last Generation console. In this preservative, games like Grand Theft Auto 5 has numerous graphics changes, especially with the textures, visuals, and others. Other than that, games can also have different frame rates and even resolutions. Games like the Last of Us, can run at 720p at 30fps on the PS3 but on the PS4, gets up to 1080p at 60fps. This can be a major change, especially if you want to remake Sonic Unleashed into the next gen consoles. Even some of the games at that rate can be almost "Photo-Realistic" and have room to spare for it.
The most changes however goes to the Wii U and Wii. The Wii does not include HD Graphics though the Wii U does and unlike the Wii, it will nearly look similar toward other next gen consoles. However, expect great changes to come if released on both the Wii and the Wii U though that part may not happen soon...
The only problem is if the Hedgehog Engine will work. It has since work on the Wii U with Sonic Lost World along with the PS3 and Xbox 360 but it never works on the Wii and this remains a question when it comes to the PS4 and Xbox One. However, expect graphics to be similar to Sonic Generations on the PS3 and Xbox 360 if a Sonic game is on that console for the last time but on the PS4 and Xbox One, similar to Lost World but better...
How much it'll be required to store
Thankfully, PlayStation has kept Blu-Rays ever since the PlayStation 3 so developers on the PS4 can be very familiar on how it's done. However, the Xbox One has finally use Blu-Ray for the first time, though it uses DVD-DL on the Xbox 360. The Wii and Wii U only uses proprietary components for it's Disc storage. One single layer for the Wii Disc is up to 4.7 GB and 8.7 GB for a dual layer. The Wii U's game disc is almost the same as a single Blu-Ray Disc at 25 GB. However, Xbox 360's is similar towards the Wii's disc. Towards how much it is, Both the Xbox 360 and Wii are similar though Xbox and PlayStation taken steps to make more space to use a game. Eventually it won't be a problem however the storage on all games will also effect on everything that is mentioned above.
Cross-BuysThis is something very important to console companies. For the latter note, The PS4 did offer Digital Upgrades for PS3 game also on the PS4, like Battlefield 4 and Call of Duty: Ghosts. Other than digital upgrades, games like Fez, and a few, mostly indie games, do offer Cross-Buys, which allows users to not only get a game on the PS3 but also on the PS4. However, its unknown if the Xbox One does it, and so will the Wii U though Nintendo has plans on it which will include the 3DS.
When it'll come and if Last Gen is included
I won't tell you if a Sonic game should come by now or soon but still, as on my last post, prototypes for the next Sonic Game is under development. It's unknown if a game like this could happen but it may not... It's possible that if games release by late 2014 to early 2015 can get multi-generation but it's the chances are getting slimmer as console companies like Sony Computer Entertainment and Microsoft is pulling the plug on support and others. Much of those years end might come by 2015-2016 when their life spans end. And the Wii is getting almost a zero chance as most of the developers are starting to not make games for the Wii as well as Nintendo's focus to the Wii U along with stopping production on the Wii. So the possibility on when the PS3 and Xbox 360 could get their last and possibly another Sonic game remains a question though the PS4, Xbox One, and possibly the Wii U will get it, but it depends if they fell they're ready to leave the last gen behind.