I think every big opponent Luffy was fighting in the past will play a big role in the end of One Piece -> Crocodile, Enel, Rob Lucci, Moria and of course Doflamingo.
Doflamingo is now imprisoned in Impel Down. So the question is, how will he come back?
I think Doflamingo has a plan.
We know that Doflamingos motivation is to become Pirate King.
If you want to become Pirate King you have to face the Yonkos, but Doflamingo is not strong enough for that. So he was supporting Kaido with artificial devil fruits “smiles”. But how will this helping him to become pirate king? That makes no sense, to make one of the Yokos more powerful. There has to be more behind this.
There is one panel that maybe give us a hint about his true motivation.
We can see Momonosuke using his smile devil fruit and he has a vision of Doflamingo. Doflamingo tells him “get aholf of yourself”. He is trying to control him.
Doflamingo has one technique called parasite, so he can control people with his strings.
What if every smile produced by him has one of this parasite strings in it. So he would be able to control Kaidos army. That was his plan from the beginning, but Luffy and Impel Down got between it.
But this trump card can be his escape-ticket from Impel Down. Doflamingo mentioned in Tsurus ship “It’s not much too ask, i’d like a newspaper every day”.
I think there is a deeper meaning behind this sentence. Doflamingo will know when the big war is coming and he will reveal his trump card against Kaido to the world government/marines. This will be his ticket out of Impel Down and return in the story. The marines will free Doflamingo so that he can take part in the war, hoping he will contribute to Kaidos fall. That will be a win win situation for both sides.
If this turns out to be true, than i‘m sure, that there is one of the parasite strings in Sabos Mera mera mo mi too.
What do you think guys?
*Theory by Wolfram87