The film is fiction. In reality, Marcus Aurelius had no such intention. In fact, Marcus made his son co-emperor in 177, 3 years before the movie begins.
Regarding why a powerful man would advocate a return to the Republic: It was a popular football that got kicked around a lot because it was popular. No one who advocated it was ever serious.
Regarding why a fictional powerful man would wait, well, that's answered in the other responses.
If polar bears can't stand the change, they should go back to where they came from.

Makes sense. Thanks.
You've got to go through Hell before you get to Heaven.

He wanted to first hand someone he trusted power who could live long enough to find and rout out the corruption that's been allowed to fester in the senate (with guys like Falco) before handing power back to them.