First of all, I would go back to 3D games. Yes, UbiArt's style is beautiful, I'm not denying that, but I think a return to 3D would increase the hype for many, many people.
I would like the game to be an open world in the Glade of the Dreams. I don't mean an open world like Rayman Revolution, which could be quite chaotic without a map, but a well designed open world, with the Glade of Dreams very alive and as colorful and magical as ever, with the possibility of interacting with the different characters that inhabit it, either to continue the story, to do secondary activities, and why not, some other mini-games. I think it would be interesting to see some RPG elements in this game, either modifying Rayman's clothes or improving different attributes of the character, such as sprint speed, swimming ability, helicopter duration... And even some more "curious" attributes, such as
his weight 
. It would also be interesting to be able to alternate the ability to shoot with the ability to throw the fist, thus combining the classic Rayman attack with the attack we saw in Rayman 2. These attack modes could be incorporated somehow in the attributes to improve the character.
I also think it would be a perfect opportunity to give some coherence to Rayman's lore. After all, that's a job that Ubisoft itself should have done back in the past, right?