EatChildren
Currently polling second in Australia's federal election (first in the Gold Coast), this feral may one day be your Bogan King.
Why not have them explore the Leviathans even more? I think they were such a mysterious addition to Mass Effect 3 and it wasn't that fleshed out.
I liked the Leviathans too and wish they had been explored more, but it's a staple of good writing in sequels/successors that are starting on fairly new ground to avoid repeating tropes and clichés of its predecessors. One of the worst mistakes writers can make is introducing a new big bad that somehow remained dormant/unknown/mysterious in the previous entries, and now all of a sudden is relevant and replicates (or worse, exceeds) the previous threat. Unfortunately the Leviathans fit this criteria; they're very similar to Reapers and after three games focusing on that threat the fourth should do whatever necessary to establish something new and original.