Dortmund is preparing for Guirassy move which would be really good to strengthen their scoring skills. However there are already 3 centre-forwards in the squad.

Füllkrug is a new transfer you know and he played really well in his first season. Together with Malen on the wing they made a really good goal contribution in total. However maybe Dortmund wants an even more productive striker like Guirassy in Füllkrug's place. When it comes to Haller he has been facing injury problems quite often so I don't feel like he has a bright future with this team anymore. Maybe he might even leave this summer.
Other than that Dortmund should work on signing some centre-backs too. Hummels is leaving and Niklas Süle has put on weight quite much recently.

This one is a bit of a sensitive one. Haller had cancer and thankfully he is healed now but still he is a player who had some injury concerns and they are fearing that it may repeat so they are keeping him, but they are not entirely sure how the season will go for him. Moukoko is a youth player, he is growing older and older every year, trying to take that place, but he is not there yet, he is not starter level great, he is an unbelievable fire starter off the bench though, so they are keeping him as option.
Fulkurg is the only one that they have, and if he leaves, they will not have a starter level striker at all. So they might be looking to offload him, for a lot of money after euro , and they may get someone else because of that.