Reading Micronet Encyclopedia edition 2008 (a spanish electronic Encyclopedia with a lot of biographies and a tool that permits to know who was living at a determinate date of history, this with the Encarta 2001 Edition that was the last Encarta that also had a tool that permitted to know who was living at a determinated date are surely two great tools for Alternate History) about Juana la Beltraneja and Isabel one could clearly realise that it seems that from the point of view of Castilla interests only France, Aragon and Portugal could be considered as possible countries where found candidates to marry Isabel or Juana, because in TTL Isabel refused to marry Fernando and remains stubbornly against any possible marriage during some years after the POD surely this will alienate some of the possible allies that she had in Castilla (like the archbishop Carrillo), this clearly mades of Juana the probable heiress of Enrique IV, because the needs of Castilla and in the case of Juana the interests of the castillian nobles that supported her, principally the marquis of Villena, remains in increase the power of Castille with a marriage with a neighbour that could give Castille military help and prestige I think that possibily Aragon would be the chosen, Naples or other italian pretender (like the mentioned Sforza, although this yet was married) would be considered territories too far and little in power compared with Aragon, Portugal and France, also a marriage with italian pretenders could mean a war against some other major power (Aragon or France).
The plaussible option so would be a marriage with some powerful neighbour, personally I would chose Fernando of Aragon.
This naturally seems that the history could repeat with Juana instead of Isabel, but Juana I think did not have the will of Isabel, surely she would be more influenciable by castillian nobles like the marquis of Villena and her new husband Fernando of Aragon.
Also if Juana and Fernando had a son that at the difference of prince Juan, the son of Isabel and Fernando that died in 1497, could survive this probably could reinforce the feeling of peninsular unity (ever naturally respecting the different laws that in the Aragon crown for example was contemplated in Valencia, Cataluña and Aragon and the prerrogatives of the castillian nobles) without the shock in the future of having a heir in the person of Charles I that provoked the rebellion of the Comuneros in Castilla against the intromision of a foreigner king (Charles I)that no doubted in favoured the flemish nobles in the government of Castilla, also the very bad economic situation of Valencia combined with the presence of a focus of black death in the region plus the fact that Charles I decided to not convoke inmediately Cortes in Valencia because was more interested in assured his imperial election that in trying to solve the problems of the kingdom of Valencia caused the rebellion of the Germanias.
All this could be avoided with Juana and Fernando married and with a surviving son, also another effect of not made Isabel queen of Castilla could be the fact that in TTL Cristobal Colon not have the so very good reception thanks in part to Isabel had his project of navigating in direction to the west to found a new route to the Indias, the will of Isabel was marked of combine the need of searching new lands in the Indias for the castillian expansion combined with bring the doctrine of Christ, with Juana and Fernando probably there would not so enthusiasm like in OTL and more pragmacy in reference to financing a project refused by other european monarchs.
Also without a Castillian civil war between Juana and Isabel, the civilian confrontation between the Zegries and the Banu Sarrach (or Abencerrajes) probably could be avoided initially: the unstable situation of Castilla in OTL made possible a fight for the power in Granada between the partidaries of continuing with the actual situation of relations with Castilla -the Banu Sarrach- and the factions that supported the brother of the king of Granada in conquering the power, also a Castilla without a civil war ( the last battle was fighted in La Albuera in february 1479) means that the granadians could think twice before making the movement of taking Zahara in 1481 that was the casus belli that the monarchs of Castilla and Aragon of OTL used to begin the hostilities against the kingdom of Granada.
In consecuence this probably could delay the Granada War a lot, Juana and Fernando could decide to intervene in a possible future civil confrontation in Granada in favour of one of the sides eventually transforming Granada in a vassal kingdom and with the payment of some important frontier populations (principally Ronda and Loja) but in TTL Granada kingdom could survive at least formally independent a lot more than in OTL.