Regarding your idea - I think it would have virtue if the entire game had been designed around that philosophy. However currently I don't think it'd work very well with the way the game and mechanics were set up.
On the one hand I like the idea because it makes the idea of a frontline & particularly Tanks much more appealing. Being a "Hard Tank" (think Braum, Malphite, hard initiation but little damage) would be so much stronger if you could actually be a true Wall between your enemies and your carries. Being able to dive in the way of incoming damage/CCs would make the Tanks so much different in terms of their role.
However it could cause a lot of problems, too. Instead of "dealing with" Mobile Champions this actually makes them worse. Imagine you are Veigar facing off against an Ahri (the epitome of an Immobile and Highly Mobile AP-Carry) and you decide to take the initiative. Flash! Q! ULTIMAAAAAATE!!
Ahri just ults behind Leona, who buffs her defenses and (having little to 0 AP) takes little damage from it. Then Ahri dashes past her and melts your face off. This would make Mobile champions able to 'force' their opponents to intercept incoming spells, and makes the life of an immobile champion even more difficult.