Entwives going West?

[Mnemosyne]

It is also possible that the ents' souls returned to Valinor. The souls - spirits? - of Elves go to Mandos, so why can't ents? They are living creatures just the same. That means that they'd have the perfect opportunity to reunite with the entwives. Maybe it's not specifically in Mandos, but somewhere in Aman. Perhaps Yavanna will make her own place to keep the souls of her creatures.

They aren't living creatures just the same. Souls are only confirmed for the Eruhini--Elves and Men. It's always seemed to me that the Valar-created speakers are fundamentally different in that respect. We get the most information for Dwarves--Durin is reincarnated in one of his children--but even that is shrouded in mystery. I don't think you can assume anything, even the existence of souls, in an Ent or a Dwarf or an Eagle. They may be sentient, but they lack the theme of Iluvatar which sets Elves and Men apart from the rest of creation.

That isn't to say that your idea couldn't happen, but it's mere speculation and the little evidence we have seems to be against it. It'd make for a wicked fan fic, though.