Learn Italian: Siamo ottimisti per il futuro. - We are optimistic about the future.

Prepositions rarely translate word-for-word. While in English you are optimistic 'about' something, in Italian you are typically ottimisti per (optimistic for) something like the future.

Since 'ottimista' ends in '-a' in the singular, why does it end in '-i' here instead of '-e'?

Adjectives ending in -ista are unique! In the singular, they are the exact same for masculine and feminine (like un ottimista or un'ottimista). However, in the plural, they take standard endings: -isti for masculine or mixed groups, and -iste for exclusively feminine groups. Here, ottimisti means the 'we' includes at least one male.

Is it wrong to say 'Noi siamo ottimisti per il futuro'?

No, it is grammatically correct! But Italian usually drops the subject pronoun (noi) because the verb siamo already tells us exactly who is doing the action. You would only add noi if you wanted to strongly emphasize that WE are optimistic (as opposed to someone else who might not be).