I didn't dislike Krasinski in Dr Strange 2 but I wasn't fully sold on him. He was fine for a cameo, but I don't think he'd pull off the character in a lead role. Mr Fantastic - or at least my interpretation of him - has always been arrogant, aloof and disconnected. It's clear he thinks he's the smartest person in the room (because he is, and probably the planet), and he's not necessarily a cold person but it's obvious he focuses more on his work than on the people around him, even if he does care about them. Krasinski just never sold me on being the smartest person on the planet, not did he really nail the slightly disconnected aspect of the character, I feel.

It's perhaps a slightly weird suggestion, but I've always felt that Glenn Howerton (Dennis from It's Always Sunny In Philadelphia) would be my ideal Mr Fantastic. He can absolutely pull of that arrogant, slightly narcissistic aspect of the character, but I feel like he can do it in a charismatic, likeable way. And he can definitely sell the idea that he's very intelligent. Plus he looks the part.