Of course there will be no Union of the Crowns. History went in a very specific way and everything will be thrown over now. I don't see this process. Sure, Aragon was under the House of Trastamara but that doesn't automatically mean Spain.
1) Spain isn't the same than Union of the Crowns. If it helps, think of Scotland and England being united but kings failing to fusion the crowns before a long time.
If we really want to talk technicalities, Spain wasn't unified before Philipp V in the early XVIII century.
2) You don't see the process, yet you know that Trastamare House managed to spawn in both realms. The OTL marriage between Isabella and Ferran isn't some sort of historical accident, but happened precisely because earlier stages were reached for this and was searched by both dynastic branches with support of at least Aragonese high nobility, clergy, etc.
History doesn't go in a "specific way", that's predestination you're talking about. History, events how they happened, were made because of several, conflicting tendencies. And the tendency to union between Aragon and Castile was a strong one, that was reinforced with time depsite backs and returns (critically during the troubles of Henri's reign, but that hardly prevented Aragon to continue to search the wedding with Ferran, or Joan II)