I guess the one thing Belf wants most for his country is for it to enjoy a similar strength and stability to what it had prior to the first stirrings of the War of Shadows (FE11). To his sense of patriotism, Grust was at its best when it was strong and respected - or perhaps, given that all we know of Ludwik is that he’s cowardly enough to use his own children to preserve his life, Belf simply wants his country to be as it was in the tales of his youth. Ludwik has to have ruled over Grust for at least 15 years given his children’s age, and it’s more likely he ruled over Grust for long than Belf was alive up until he died after FE11.
More personally, Belf, being the sentimental guy he is, would wish for a time in the future when his old (BSFE) unit - which includes Robert, Leiden, Camus, and Nyna - could reunite once more, even if just for a day. For a few years after FE12, Belf adheres to the promise he made to Sirius to not tell his friends the truth, but the hole made by the final two’s absence (compounded by the loss of other old teachers/allies like Lorenz), and more precisely his friends’ noticing the effect it had on him, eventually make him break that promise. I haven’t discussed with Ree whether, after his A conversation with Sirius, Sirius would be willing to tell Belf where he went after “departing Archanea” (could go either way, but I unfortunately suspect the actual answer would be no), but IF the answer were yes, Belf would undoubtedly want to take Robert and Leiden to Rigel someday after things have settled down in Archanea.
