What did Madoka wish for in the original timeline?
She wished the cat in the opening (named Amy) to be saved from being run over by a car. This is revealed in the Drama CD 1.

More info about wishes in the series: https://wiki.puella-magi.net/Wish_theories#.E2.9C.94.C2.A0Timeline_1_-_Save_that_cat.21

More info about the Drama CD 1: https://wiki.puella-magi.net/Drama_CD_1