Well, to be precise, it seems that Isaakios I didn't nominate Constantine Doukas as his successor from the first moment. Psellos convinced Isaakios to abdicate and become a monk in order to live what was probably considered to be his final days, in relative peace, but the succession wasn't ironed out. Psellos most likely suggested Constantine Doukas, with whose family he maintained relatively close ties thanks to their positions at the top of the Constantinopolitan elite and his position as tutor of his son, Michael; but Isaakios' wife, Ekaterine of Bulgaria, tried hard to dissuade the emperor from abdicating. It was only when it became clear the emperor was determined to let go of the title of emperor, and Isaakios' brother Ioannes, refused to take the throne, that she changed strategy and tried instead to influence the selection of the succession, most likely with the goal of ensuring that the next emperor would be someone she could work with and help protect the influence of the family. In this context, the choice of Constantine Doukas made sense - he was seen as relatively inoffensive and it would allow her to co-opt Psellos for the time being; in fact, it seems that in the early months of Constantine X's reign, Ekaterine did play an important role in government, perhaps as part of an agreement with Isaakios in exchange for his agreement to become a monk.
Therefore, we could say there's a lot to play with: Isaakios snapping at Psellos, Isaakios deciding that his duty to the empire was above personal doubts etc. I think that under these circumstances, since Manuel wouldn't be around, the best option would be Ioannes Komnenos: Domestic of the Schools at the time, and with at least two living sons, he offered the best chances for the continuation of the Komnenoi hold on the throne. It wouldn't be difficult for him to consolidate his position; the support of the Doukai could be secured with a matrimonial link like the Alexius - Eirene match of OTL (perhaps a match between Manuel, Ioannes' eldest son, and Anna Doukaina), and opposition from the Church epitomised in the ambitions of Keroularios would most likely be less severe, which could make reform plans easier to implement.
As for the kind of policies that would be implemented, these would most likely be a continuation and expansion of the mesures Isaakios took. Therefore, revocation of proniae and grants, restoring the effectiveness of the taxation system, perhaps efforts to contain the debasement of the currency, and continuation of the reorganisation of the army. Another good thing is that if Ioannes dies at roughly the same time as IOTL (ie 1066 - 1067), he's going to have a smoother succession, with Manuel being 20 years old, perhaps already acquainted with the army, and with links to at least another major aristocratic family with a strong presence in the capital; therefore, unlike the OTL's vicissitudes that characterised the reign of Romanos IV, there could be a stabler political environment, allowing the empire to prosecute the war against the Seljuks more effectively.