Atalanta need to stop selling their important players like they sell Hojlund last season. They will play in Champions League next season, they will need to upgrade their squad depth to able to compete with top teams. Gasperini is a good tactician, he elevate Atalanta to be a tough team and have an attractive gameplay throughout his coaching career at Atalanta.
Apart from not needing to throw away the old players that Atalanta already has this season, the Atalanta team itself also needs to look at other players who are much better if they have enough funds to buy new players so they can be included in the existing squad. I am also starting to be impressed with the gameplay created by Gasperini because this was very visible when Atalanta played in the Europa League this season and also in Serie A this season. Hopefully next season things like that can continue with Atalanta so that other teams don't think of them as a weak team.
I don't know if it was from my own point of view from this season domestic and international competitions that Atalanta can only perform very well in Europa and not in their domestic leagues. Atalanta will still be finding it difficult to challenge clubs like Inter and Milan who are always fit for domestic leagues. However, when it comes to European competition, Atalanta can do better than playing in their domestic league because their opponents don't understand their game pattern and are not used to playing Atalanta.
This is why you see that it was very easy for Atalanta to defeat Liverpool and Leverkusen just like that because they all have their own game patterns but that of Atalanta surpasses both Liverpool and Leverkusen. Like you said, Alonso needs some new players to strengthen the club because they are playing in the champions league next season which is not always an easy league, because every club in UCL are champions.