i'm okay with a crossover SMG3. i'm sure Wii U can handle a SMG3.

I enjoy the gameplay of SM3DW more, but the experience of SMG was simply better due to the theme, presentation and style, not to mention the trippy, unique gameplay. I would enjoy a return to either style, but I would prefer a hybrid, Super Mario Universe:

-Classic powerups such as Fire Flowers, etc, and classic life system where damage takes away power and shrinks you (no life bars).
-Standard attack is stomp from above and punch block from below. No spin move.
-An interesting and unique theme and presentation, more in tune with Galaxy than 3DW.
-Ideally you would progress through a series of "hub worlds", where you can roam freely and explore (N64ish style). This would involve a behind the back camera and free looking. There would be action and platforming and challenges, but mostly a more relaxed feel and an exploration focus. Find hidden stars, etc, to progress. But then within each hub world there would be a series of courses. These would be more "classic Mario" style, and would use typically a more fixed camera for precise gameplay and tougher platforming and action challenges. They may or may not have a time limit. Most would need to be completed to progress to the next hub world, though finding hidden stars in the free roaming section might enable you to skip some of them if you're not interested in 100%. While many would have a 3DWorld feel, others could have a more Galaxy-like feel and crazy twists. 0 gravity or planetoids included. You never know what you're going to get when you enter a world course.
-The main game would be primarily single-player focused, but online 2-player or perhaps split screen/controller screen two-player could be an option too.
-Online multiplayer vs matches would be available to add value and a unique experience. For the first time, true "vs Mario Platfoming". Cool multiplayer maps based on the classic Mario universe, where you battle each other using Mario Powerups. Kill your enemies by jumping on them, bumping them from below, picking them up and tossing them into a pit or shooting/whacking them using power ups. All kinds of playable characters including Peach, Toad, Koopa, etc. Total mayhem. Total fun. This would essentially replace the more chaotic multiplayer in SM3DW.

I know two person who stopped playing Terraria because of the difficulty.

You will be surprised.

You do have somewhat of a point, because Galaxy is as close to perfection as gaming gets, and in my opinion probably the greatest game ever created. So yes, I would love a SMG3, and they don't need to reinvent the wheel.

On the other hand, the fact that Nintendo is constantly changing up the formula of its games is part of what makes them so great. I'd say they already struck gold with Mario 64, but instead of making 64-2, 64-3, 64-3 etc. they went in different directions and we endup with Sunshine and then Galaxy. Same story with Zelda.