the first thing that comes to mind is some sort of ancient super weapon hidden for eternity and you'll be able to use it to destroy the reapers. in the 3rd or second.

maybe an even more powerful ship than sovereign will join.

or maybe theres a control panel or something that drives all the reapers, either on a mother ship or in the dimension in which the reapers came. - doubt it though

you'll probably have to go to the universe where the reapers came

i don't think there will be direct fighting with the reapers. probably cut scenes showing them die.

there might be an all powerful reaper squished to a human form.