If Bryan Cranston joins the MCU, they should introduce him as Norman Osbourne. But have him form the sinister six. Steal/craft a suit similar to iron man's (Iron Patriot). Or form the Dark Avengers (needing to form a contract with Fox for Daken and Wolverine). There's a lot Norman Osbourne's character could bring to the table. Even without becoming the Green Goblin.
In my honest opinion, if they were to bring up the Green Goblin. And this is all THEORETICAL. They would be best to pick up from Civil War. If Crossbones kills Cap at the end, Oscorp would get a sample of Steve's blood. Harnessing the syrum that gave Steve Rogers his super soldier capabilities. However, due to failed attempts and tampering of the syrum. Osbourne tries it on himself. Now this is where we're reminded of the doctor's words, when he tell Steve that the syrum will only work correctly on a "good man". This is what would turn Norman Osbourne into the Green Goblin. And imm talking about the actual Goblin looking one. Throwing fireballs like a hell spawned demon.
That would be the only logical and actually good way to bring in Green Goblin. But that's just my thought.