If Retro is making a new Metroid then I would want it to be a reboot of the series much in the same way that the recent Tomb Raider rebooted that series. It would be nice if it was a third person action game similar to Tomb Raider. Lastly, Retro, or Wayforward, could make a 2D side scrolling sequel on the 3DS to compliment the Wii U game.
Other M is such a bastardization of Metroid that it practically was a reboot. It sure didn't feel like it fit with the rest of the series.
I really want something "quest" based. Do you get what I mean? Like some games have some sort of quest to them. You're some character and you go out to accomplish something in some videogame world. It doesn't have to have a great story but there is some narrative. The main Mario, Metroid, Zelda and Pokemon series all all of this type. Your character goes on some quest whether it be though sequential levels or an open world. Mario Kart, and racing games in general, don't have that, even if they have characters. It's more like a sport. The Wii Series does not have that. Tetris doesn't have that. Fighting and rhythm games don't have that. Those game are effectively pure gameplay.
There is a place for games like that but I find I get bored of those series as they don't really have new places to see. Quest games can always give you the same gameplay in a different world and seeing the world can often overcome derivative gameplay and still feel like a new experience. A new track in Mario Kart of F-Zero isn't enough to do that and it's hard to update the gameplay.
So while I would prefer a new IP I'm fine with:
Mario platformer or RPG
Zelda
Metroid
Star Fox
Kirby
Fire Emblem
Pikmin
Kid Icarus
But not with:
Mario Kart
F-Zero
Wii Series
SSB
You get the idea. I feel a "quest" game is the only way to really get any personal creativity from Retro. Unless they really came up with some gameplay ideas, current Mario Kart games could be made by any group of code monkeys under Nintendo's supervision.
Though ultimately I find that I'm not impressed by games that I figure Nintendo would have given me anyway. If I can say "if you didn't make it, someone else would have" then it's just not that impressive.