Here's my own personal spin on the "Frozen Wastes/Wilds." We've had now two versions of skeleton strongholds. The legacy ones we known long since launch, and the ships. We have to a new opportunity if both ice-sheets and icebergs are introduced.
Fortified Bergs! The idea is simple, but can be expanded upon. Like skeleton ships the bergs can move freely about the ocean, but perhaps only within the "Wastes." the bergs drift about slowly, perhaps by the wind, or currents that simply aren't a mechanic(perhaps that semantic doesn't matter). Either way the bergs can come in at least a few varieties with all different manner of loot aboard.
Instead of fighting waves of skeletons everything you need fight has already spawned, and you need to simply make your way up towards a pinnacle, or a descent to it's abyss. Maybe some have gated areas, or 'tiers' each with it's own vanguard/sentinel/captain that you must clear in order to pass onward.
These could vary in difficulty, and provide a fun challenge to those willing to take them on. Enemy health, type, amount of cannons mounted to it, and so on. Lil'floating fortresses of chaos, sitting there for us to blitzkrieg our way into.
Tl;dr big bag floaty forts.