You don't even need to avoid the Iberian Wedding. After Isabella died Ferdinand re-married and tried to get a son who he could leave Aragon to, just have this succeed and the Kingdoms won't be united.
There's already the Habsburg Marriage between Joanna and Philip at this point, so Castille will still be going to the Habsburgs, but the Habsburgs will be more likely to recognize Aragon in return for support against France.
At this point Aragon is already Ruler of a decent portion of the Mediterranean, including Sicily, with a claim on Naples that would historically be resolved in their favor and I see no reason why that wouldn't also happen here. Aragon supports Imperial Wars against France in return for being supported in their claim on Naples.
Getting Northern Italy may be more difficult because Spain seized that by virtue of Charles V being Emperor, which is a claim Aragon won't have, but there's always time for various Marriages to happen.