To be entirely accurate, Etemon didn't fuse with Datamon. He fused with his own Dark Network and became a ball of flying death. Which could totally be a capacitor thing for the bottom right island! Collect 20 Dark Network Capacitors and you get your own hulked out flying monkey! Do want~
I also second the idea of Mammon->SkullMammon, as I was rather boggled when I first saw that wasn't a thing.
Now, for fresh suggestions...
-Lunamon->Lekismon->Crescemon->Dianamon COME ON!! Coronamon goes up to an SP, but Lunamon gets blocked at a Rare that isn't even her final form? I call shenanigans! Shenanigans of the highest degree! The moon deserves just as much love as the sun! (More, actually, but hey, one step at a time~)
-Candmon->Meramon->Deathmeramon->Boltmon More love for the dark! This is another line that seems to needlessly stop short of its' true final stage.
-Impmon->Wizarmon->Mistymon->Dynasmon One of the few Royal Knights you can't get outside of DM, far as I'm aware. This alternative branch for Wizarmon would resolve that nicely!