3. Does anyone remember Iron Lad?
Back in 2005, Marvel released a comic titled Young Avengers which featured a team of characters that were, essentially, young people who looked like famous Avengers (all with fun twists) including a kid named Iron Lad who had what looked like a thinner version of Iron Man's old silver and red armor on.
While many of the Young Avengers had really interesting twists like two of them being the magically created children of Scarlet Witch or one of them secretly being a bisexual space prince, the biggest reveal came from Iron Lad who was.
Turns out that when Kang was younger some bullies attacked him, sending him to the hospital. Kang took his younger self back in time hoping to convince him to kill the bully instead. The younger self was shown all the atrocities Kang would commit and instead of being inspired by them like Kang hoped for, he stole time travel tech and a bio-metal suit that obeys thoughts and went back in time to stop Kang.
There is a moment however where he succeeds and it's bad for everyone. In killing his older self it creates this butterfly effect where some of the younger heroes never exist. Different people take different roles, some people are dead now, and Jessica Jones loses her baby. It's not good.
In order to fix the timeline, Iron Lad realizes that Kang needs to exist, but he kills Kang. So the only way to fix the timeline is to fulfill the prophecy and go back in time and become Kang himself so the correct events happen.
This would be a really interesting way to introduce a younger actor and have him be a big bad by having Iron Lad give in to his fate.