Concept the Third...Fourth? Unfortunately, at least part of what I thought I was clever in considering for Concept the Third is not working in the fashion I'd expect, so I'll be eyeing the thread.
To whit, I love the flavor (if not necessarily the synergy) of combining Sundered Lands and Forsaken Wastes. They're both bad, bad crews but seem to have a greater depth than (ironically) the Underdepths, which feels like a faction of mean dudes and sexy mean ladies. The Sundered Lands represent an almost zerg-like cultural expansion, continually welcoming new races into their collective; Draksar Berserkers and Draksar Psions being my two favorite examples, though I'm looking forward to Lonx and Yeti flavors. The Forsaken Wastes, meanwhile, have such an elegiac quality to many of their runes, and mechanically they can be brutal. The synergy doesn't always stack up, however, since the factions are on opposite poles in terms of central thesis.
Sundered Lands runes, and their faction bonus, tend to focus on longevity. High defense and regeneration keep a particular creature in the fight for at least a few rounds longer than his cost might indicate, and the brutal area spells the faction possesses allow it to win wars of attrition by simply standing after the other guy has fallen. Many of the runes are expensive, given all the defensive qualities stitched into their designs, but their ability to slog through lesser runes (especially in a full-faction) justify that.
The Forsaken Wastes, by contrast, die a lot. In fact, most of their runes were dead before you cast them. Traditionally FW runes have been relatively fragile, because the faction bonus and design of many creatures and spells is all about recovering slain runes quickly, rather than agonizing over keeping them alive. There's also a disease thread which was popular for a while, but combining disease and another faction tends to be very difficult (though one could probably do something with Ironfist Constructs and Medics) because even if your other faction isn't suffering much damage from disease effects, the stat reduction from disease is bad enough. Similarly, the good FW relics penalize non-undead creatures often as not. Luckily, I don't have any of said relics.
Nonetheless, I think careful rune selection can yield a very profitable balance between the two factions. Besides obvious inclusions like the Draksar Doomslayer (which has Ally: FW trainable) and the Temple Guardian (same, plus bodyguard to hedge somewhat on FW fragility) the main rune I'm interested in combining with FW is the Arthomancer, since his Hive laying ability dovetails wonderfully with the Mason's Spire. Or would, if it was working correctly. Working correctly, the Spire would boost each lain Hive's hp by 10, counteracting the 1/2 hp limit on lain Hives almost entirely. It would also provide a nora refund for the Hive, which turns Arthomancers into limited nora generation engines not far below a Deepcave Extractor or Elvish Artisan. The Hives themselves do minimal damage to the FW creatures in the deck, since creatures with Boon of the Undead have significant poison resistance.
There are also spells in each faction which grant favored race to any target creature, so I could be turning my Draksar/Skeezick into undead (to grant them disease immunity) and my undead into Draksar for Draconic Force and Hive immunity. With 2 Arthomancers and 2 maindeck Hives, plus Mason's Spire and Crypt Guardian, there'd be a lot of battlefield control at a relatively cheap price. The Doomslayers provide excellent font contesting, particularly backed up by Essence Drains to clear defenders. Both factions have powerful rare-level spells that grant extra attacks (Quickening and Mobilization/Draconic Benediction, natch) as well, meaning that the deck could provide both stable attrition and extremely swingy rounds.
Finally, if I hew Sundered Lands for my Shrine I get a regenerating Avatar, which should help to counteract the shrine life loss of Skeletal Berserkers, Executioners, and/or Blood fiends. With a toolbox of creatures and damage types available on both sides of the fence, this deck would be highly technical to build but quite likely resilient and abusive on the field.
Bonus! I'd also like to put something together abusing the Sundered Lands spell Sandspout and the K'thir Forest spell Reclaim. I'm not sure I've ever combined those two factions before, and Garu/Skeezick springs to mind as a resilient, melee-focused team that would greatly benefit from enemies being incapable of making attacks for a dozen or so turns a game.