Mega Man X: Make an alternate universe 'reboot' that keeps the more anime-esque style (with the fancier newer designs shown more recently, like in the TruForce X figures) but with a slightly more grounded setting (emphasis on "slightly") - think Ultimate Spider-Man (but not the rest of Ultimate Marvel, which basically was "everything is 'realistic' and everyone except Spidey is an asshole"). The story should have a greater connection to the 'classic' era of Mega Man (though in this setting would be different than the actual classic games, it's an alternate universe, we can do what we want here), with Kalinka Cossack taking Dr. Cain's role and having succeeded her father (who has retired) as an accomplished robotics expert, with X having been awoken 30 years after his creation, right on schedule. Expand the Maverick Hunters and include some new animal-type reploids who are actually allies, and actually have humans who aren't Dr. Cain actually show up in the story. Oh, and finish the damn story, and not on a fucking sour note that is the implication that humanity and robots are doomed to eternal conflict until the apocalypse happens (as implied by MMZ and MMZX, what with Legends coming after it).
As for gameplay... Copy the core mechanics from Warframe. No, I'm serious, Warframe is basically a Mega Man X game in all but name and looks, right down to the wall-jumping. Replace sprinting with dashing and replace crouch-jumping with air dashing, and modify the weapons to be more Mega Man X-like, and you've got a winner.
Sonic: Same 'reboot' approach as my MMX idea - still generally kid-friendly, but with a more fleshed-out world with a tone and style akin to Avatar: The Last Airbender that's closer to the classic style but with just a bit of 'modern' influence. I'd just get Tyson Hesse to do the designs, really. Provide an explanation for how the animal characters seemingly have superhuman abilities with the 'Mobius Blitz', a more flexible version of the Speed Force from the Flash comics, with Sonic having been born with an innate connection to the Blitz, being capable of Flash-like speeds if he really puts his mind to it. Give Sonic and Eggman something of a history before they become enemies, something like the classic US Sonic 'bible' story with the pair gathering the Chaos Emeralds together, but with Eggman becoming evil only after losing himself to the temptation of using the power of the Chaos Emeralds to take over the world and "show them all" after years of family humiliation after the ARK incident, causing a rift between the pair and a tragic "we used to be friends" vibe with their antagonism, with Sonic hoping one day he can redeem his former friend.
Side-note with the story: I'd either make Princess Elise a 'mobian' or give her a human romantic interest who uses Vanquish-esque power armor and acts as her bodyguard. He's the one who gets killed and revived during the events of the Sonic '06 story, with Sonic basically being his wingman.
As for gameplay, the Sonic Utopia fan-game is probably a good base to work on (though with more focused level design). Focus on solid physics and a heavy emphasis on flow and momentum, with a classic-style philosophy to level design, and take advantage of the nature of the 3D camera to allow players to anticipate obstacles and opportunities to gain speed and reach new routes. The homing attack should have very limited range and mainly be a means to allow players to bounce off enemies while maintaining momentum but with significantly more leeway at high speeds, possibly with longer range the more speed Sonic is moving at. No boost nonsense - you want players to feel like they accomplished their speed most of the time. Also, optionally, integrate more refined versions of the parkour mechanics from Lost World that utilise momentum and speed, activated simply by jumping at a wall and pressing the jump button again in mid-air.
Also, as for controls, put the jump button and spin-dash/crouch on one of the shoulders. Yes, I know, seems like heresy, but barring console controllers making back paddles standard, it's the only way to make a game with jumping work while still retaining camera control (outside of using mouse and keyboard). I've found 3D Sonic works best when the player has total and fine control over the camera.