Almost correct! :) Indeed, you need an adjective here, so using περισσότερο is the way to go. The minor mistake is that περισσότερα is only the neutral adjective in plural rather than the adverb. The adverb is still περισσότερο. (It's not obvious though, I had to confirm it myself before typing the answer. See usage for the adverb here.)
This is how you form could, would etc. : Θα + παρατατικός (pats continuous)
Θα is combined with any verb a) to describe an action that is not realised, either in the past, present or future, e.g. 'Νόμιζα ότι θα έφευγε σήμερα' - 'I thought he would be leaving (=was leaving) today' or b) to express possibility, e.g. 'Θα ήταν βαρετό να περιμένεις' - 'It would be boring to wait'.