To the North is the Rime King and his Palace of Jewels. Between the Forlorn North and him, a glacier of frost wights, bore bugs, sabre-toothed ice bears. Not many non-combat encounters. Lots of harsh conditions.
To the West is a frozen ocean. PCs could boat through it in the summer, or sled on it in the winter, as long as they expect the ice to melt and plan accordingly.
Eastwards, tundra and taiga give way to volcanoes. Pterodactyls loft through the skies, and Hyperborean ruins still dot the lands. In an ancient castle, the Perilous League houses a local planetary chapter. Elementals of fire and earth rage against one another. Probably some Meks or Castle-bred or Aetherians seek to ally themselves with one set of elementals or another, possibly to have some kind of proxy war.
To the South, I see the Sanctum Sanctorum's Last Castle supplement for DCC coming in handy. Meks and Aristocrats and Freemen, fighting it out. Also, possibly, Aetherians as presented in Crawling Under a Broken Moon, number 5.
Even Further North/South/East/West: I'll probably start making stuff from Mutant Crawl Classics. Hopefully I have it by the time the PCs go that far.
To create an Encounter Table, you also need Regional Assholes
I'm taking this out of An Echo, Resounding, because I steal all my ideas from Kevin Crawford. Basically, if you want to unleash the PCs in a sandbox, you want it to already have some regional assholes - people or things that are making life worse for everyone, people who the PCs might hear about, but they sure as hell feel the impact of the douchers being around. You don't want them to reach level 5 and suddenly hear about some warlord with an army - it's going to feel like the warlord teleported in, which, if true, that's fine. But otherwise it's going to be obvious you're gating your world to the PCs level.
From Frozen in Time, I have some regional assholes and some others I'm making up.
Mutant Frost Giant, 'Mammoth Master.' He's big, he's a frost giant, he has +21 to hit and will fucking eat you. He's trapping mammoths so the tribes are getting hungry; he wants to rule the region and is extorting people by messing with the food supply. Also he wants the nomads to fight among themselves so the meanest, fightiest rise to the top and work for him. He has an enslaved cyclops ghost-binder, hill giant lieutenants, nomadic tribal quislings who figure they'll rule some hill as proxies. And a fort/dungeon/mammoth pen. He's the biggest bad. PCs are going to start knowing their people need food badly. (Taken from FiT.)
Odobenmen Enslaver, Technology Quester. Someone or some thing is enslaving odobenmen, the walrus-men of the Forlorn North, and forcing them to search ancient hyperborean ruins for... something. Lots of families and walrus-kids are hostages. Odobenmen are traditionally not friendly with the human and demihuman tribes, but PCs should find both enslaved, mind-controlled Odobenmen and fleeing, harried refugees begging for help. (Taken from FiT.)
Krib Mountain Cultists. Something slumbers in the Krib Mountains, and the Krib Mountain Cultists are trying to wake it. They raid tribesfolk for sacrifices and slaves, often disguised as traveling merchants. They have Hollow Ones in their ranks. Whatever they seek to awaken will wreck havoc on the Forlorn North. (Sleeping Elder God implied heavily in FiT.)
Then you need some people who are less powerful, but still people or things those in the region would hear about.
Broken Tooth, Warlord. A failed noble exiled from the Last Castle, Broken Tooth is amassing warriors to take the fight to the South. He's not above conscripting and press-ganging, though he'll probably present a good face to the PCs. At first. He has airs of being above everyone, somewhat belied by shattered front teeth. People learn quickly not to discuss this feature.
Man-thinking smilodons. The peoples of the North are used to the saber-toothed cats - fearsome predators, but they tend to hunt alone, and take those who wander alone. They rarely approach groups of humans but for the direst circumstances. Something has changed. Gossip portrays of packs of smilodons, speaking to one another in sibilant, coarse language. Your people say that the Warfire tribe was decimated by such, with few survivors fleeing into the night. On the wind, the refugees could hear the mocking laughter of smilodons.
Temple of Asara, Goddess of Peace. The pilgrims and crusaders of Asara have come to the Forlorn North to battle the adherents of Crom, which is a lot of the tribesfolk. Asara desires not the peace of families or friends, but the peace of nothingness, and no one to regard it. Humans are a plague against this, and the Forlorn North has many who strive against such, who have eaked out life for generations without the tender ministrations of Asara. Her crusaders carry steel maces, often dipped in pitch, and light them on fire as they quietly rush spear-wielding warriors. The roughshod wooden temple is a fortress, attempting to stop a major merchant route from bringing goods to the Northfolk.
Earth Elemental Recruiters. They have shattered an Eastward forest on their way to the Krib Mountains, and will probably crush more underfoot, seeking allies to arouse in their wars to the East. Few in number, but quite disruptive to hunters and gatherers.
Mek FOB. From the South, the once-enslaved bug-men of the Castled Nobles have marched on the North. For now they content themselves to invade ancestral structures and Hyperborean ruins. They have taken over a traditional meeting place of Northfolk, Moothill, where tribes (and goblins and ogres) could come together in peace and trade words. The Meks drove out the Windwarders and their guests, and seem to be disassembling parts of the ancient tower.