Pehaps he could become the Shogun after the final voyage of the Strawhats, and until he grow up, Hiyori and the Scabbards could take care of rebuild Wano Country.
Seems that wano people will be a key allies for the Strawhats in the "final war", so Momo being in the crew could close even more the ties of the Strawhats with Wano.
And Momo could learn the language of the poneglyphs with Robin, what would be very meaningfull as he is the heir of the original creators of the stones.
I feel Momo is the type of person who wishes to follow after his father footsteps
And by that, I feel he would be someone who really wants to be "worthy" of shogun position before ruling...
so he himself I feel would want to accomplish some stuff that can be considered as "dream" of his:
1- Because his father didn't have chance to teach him poneglyph writing... I feel that he would be someone who wants to learn the language from Robin to continue the legacy of Kozoki, so his dream might be him learning the language during his voyage from Robin, and graduates as a scholar, which will be a beautiful tie to Nico Olivia's last words to Robin by asking her to transport the knowledge into new generation...
2- Naturally when he travels, he will know the true secrets of the world like his father by reaching laugh tale...
3- Accompanying Zoro and Luffy will for sure make him get stronger and more worthy for shogun position by end of journey
Also, one last note, Oda can play around and give some real threat to Robin or Momo's life from enemies
Meaning... Robin now is protected by plot that she's only person who can read the poneglyph... I'm not asking for her to die... I'm just saying that Oda can play with our emotions and tease it about the possibility of her being in danger od death...
So... it's just for drama intensity... makes shit more dramatic and fun
At same time, I don't want Momo to join as a kid, so if an ability will make him grow as adult... sure, but if not, then no he better stay away... I want worthy people in the ship... not useless characters