Since I mentioned a while back that at this moment in time there are no reasonable suggestions for where Carrot could actually go if she doesn't join the crew, I'll at least provide my own idea for Yamao post Wano.
I always thought about the opening of Wano's borders as a single event. Something that would be done at the end of Wano and then that would be it. But after understanding more about the dawn and looking at the actual purpose of Wano opening its borders, it's more about working with others and cooperating instead of doing things solo (something that ties back into Oden's character).
So whilst the physical borders will be open by the end of the arc, outsiders won't just automatically start flooding in, as if by magic. Wano still would've been isolated from the rest of the world for 800 years. It would have no connections, regardless of the status of the borders. And so after first opening the borders, Wano needs to further reestablish its bonds with the outside world. Momo as both the shogun of Wano and a Kozuki is really the only one for the job.
I see Momo's first role after Wano to be to travel to other nations, reconnect severed bonds and to ask for their aid in the fight against the WG. I think Yamao will be Momo's guardian in this journey. But what would be the point in any of this happening? Multiple reasons:
1. Prove Momo's worth as a shogun by personally taking up the task of reconnecting Wano
2. Show the people of Wano that they need not be afraid of the outside world
3. Explain why Yamao thinks Momo will guide the world to the dawn
4. Show Momo not blaming Yamao for her father's sins like Wano citizens did before (the main thing that led to Orochi and Kanjuro becoming villains)
5. It would combat Wano's sexism by having a woman as his guardian
6. It would allow Yamao to be her own person whilst still carrying out Oden's will
7. Yamao could protect Momo like she swore she would die to do if necessary
8. Yamao can travel and explore by visiting many different nations
9. Yamao and Momo can make proper use of Oden's journal through encouraging nations on why they should fight
10. It gives them both an active role in the series post Wano
Tama might even leave with them too since just like Yamao, after bonding with Ace she too wanted to travel out to sea. In addition she's also expressed a desire to protect Momo and longs to be a ninja, a combat position. Whilst she'd be a liability in a fight against the BB pirates like the straw hats will be going into, on a mission like the one I proposed her powers would be very versatile and useful. There are also the theories of her being a kurozumi so who knows. Many of the points for Yamao also apply for her. Meanwhile the scabbards who are battered from the decades of hard service can relax by having the easier responsibility of rebuilding and protecting Wano, entrusting Momo to Yamao whilst they're on their expedition.
Feel free to point out the things I may have overlooked.