Elesh Norn, Grand Cenobite is pretty nutso; she was a first-pick bomb that was played in every deck that could play her over here, and flipped games in short order, even in my super-powerful first cube iteration that was sickeningly powermaxed. I think adding her to anything short of a powermax list is probably a mistake made to bait players into white, when the issues with the color are likely built into the foundation and just need a wholesale revisiting.
Sheoldred, Whispering One is quite potent, but she's significantly more fair than Ellie. If you support fast reanimator, she's quite absurd, but I think your environment can probably contain her well enough if you pack a solid removal suite. If you're looking for a non-ETB piece of beef for
ramp decks/reanimator decks, she's your gal. Super sweet design.
Avalanche Riders is extremely unfun and too easily degenerate, and I strongly caution against his inclusion, because he's miserable to play against. There's too much combo potential with sac outlets and recursion, allowing him to shut an opponent out of games in a consistent, boring fashion. The fact that he self-bins means it takes very little work to break him. I ran him for a while, and he was always either "quite strong" or "game-ruining". I think I can attribute the worst match of my cube's long and sordid past to spamming Rider to wipe out my opponent's lands. I hate, hate, hate that card.
On the totally opposite end of the spectrum,
Pyromancer's Goggles is fantastic, and leads to a lot of memorable games. It was very successful in my stronger list as an exciting reason to play Not-Aggro in red, and was a highly-picked buildaround that never felt boring or unfair. I recommend an absolute minimum of 23 targets in a 360 (scale that up to whatever size you cube at) to see it consistently maindecked; dip too far below that, and it's just not going to get played often in any moderately powerful environment due to opportunity cost. You can cheat by running some multicolor spells that touch red, of course, and that's really the best move to push up your instant/sorcery counts for it. If you got this route, you'd want to center those in one or two splash colors for maximum effectiveness; I think it works great for a Grixis spells angle, with something like
Izzet Charm,
Electrolyze,
Kolaghan's Command, and
Fire Covenant/
Terminate in the multi section. Of course, you could also focus on a Jeskai angle instead, leaning on
Enlightened Tutor to get it out of the deck a little more frequently, with
Boros Charm and
Lightning Helix as signposts and flexible tools that other decks would also like to have.
The other two I have no experience with in cube, but
Crackling Doom looks really boring and kind of an easy wheel to greedy control players, who are probably fed enough goodies already.
