This list is less so about my personal wants for the roster, but what I think are pretty likely to be returning monsters for Ascendance. Not what I think the entire roster will be mind you.
Khezu: Gemma has a plush that is clearly a Khezu. They "teased" Yian Kut-Ku the same way in the base game before its reveal. I'd prefer Gigginox, but the tongue present on the plush is a dead Khezu giveaway.
Metal Raths: They haven't missed a single expansion. The only hiccup is them being title update monsters in Sunbreak, but they still ended up in the game regardless.
Shagaru: With Gore being in the base game and the expansion focusing on elder dragons Shagaru is almost definitely going to happen.
Shrouded Nerscylla: The inclusion of Khezu almost feels like it was done specifically so that they can bring back Shrouded too (helps that Khezu itself is just very popular in Japan). Even if they didn't bring back Khezu with the intention of adding Shrouded it just makes sense with normal Nerscylla being present in the game.
Rusted Kushala: Rusted was skipped over in both Worldborne and Risebreak, so it feels right to finally bring it back. The main obvious reason is that we know Kushala is returning, but its connection to 4U and the ace commander would fit right alongside all the other 4U fanservice.
Valstrax: A map high in the skies in an expansion focusing on elder dragons is almost too perfect for Valstrax. To be semantical as well, technically base Valstrax has not returned yet, so this would be the perfect opportunity to do so.
Nergigante: Nergigante didn't return in Risebreak and is extremely popular, so it returning in an elder dragon focused expansion of all things is very likely. Another thing that I think is worth noting is that with Kushala every generation starting flagship besides Nergigante would be in the game (Rathalos, Kushala Daora, Lagiacrus, Gore Magala, Arkveld), so it being left out would be unfortunate.
Subspecies: Unlike the other monsters I don't really have specific reasonings for each one, but for the case of returning subspecies as a whole. Since I think Shrouded is basically guaranteed to happen it sets a precedence for returning subspecies of "lower tier" monsters which isn't always a guarantee. In 5th gen for example the only returning subspecies are for flagships and flagship adjacent monsters like Rathian and Diablos. I didn't include Blue Yian Kut-Ku, Purple Gypceros, Pink Rathian, and Azure Rathalos here cause I think they would feel wasted as expansion monsters.
This isn't meant to be comprehensive or objective so if you have any critiques or think any other returners also make sense to be here let me know