Let's talk colorless cards! The base Sudden Death cube had four colorless cards, which I kinda tried to match up thematically but didn't have great analogues.
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1.) Environmental Sciences: This is the one original colorless card I did have on hand, and slotted it right in. Two mana for two life and "fixing" is a pretty versatile midgame play. It's expensive at 2 mana, but the life gain helps mitigate the risk in playing it while on the back foot. Aggressive decks probably don't want it, except for black lifegain/pain which hasn't come together in testing but is an archetype I'm still interested in trying to support.
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2.) Feldon's Cane: The cane seems like a super interesting and versatile wincon. I like the idea of grindy decks winning by decking their opponent, so I have a Grindclock in right now to enable mill. The play patterns are pretty interesting, it can be a risky play on turn two against an aggressor, and deciding whether to start milling or add a second charge have been pretty neat. Assuming your opponent doesn't draw any extra cards, here are the turns where your opponent will draw into an empty library (whether grindstone is on the play/draw, turn grindclock comes down, number of charges before you start milling):
*note, in a natural game with no draw/mill/mull, the player will lose to decking with their turn 9 draw.
(2, play, 1): 6
(2, Play, 2): 5
(2, Draw, 1): 7
(2, Draw, 2): 6
(3, play, 1): 6
(3, Play, 2): 6
(3, Draw, 1): 7
(3, Draw, 2): 7
Grindclock is clearly better on the play and if it gets down early, but on the draw it does turn the tables and stop you from losing to decking (and enable some other card draw cards). Players should also think carefully about mulligans as a defensive pre-game play against an opponent who looks like they drafted a grindclock deck. Grindclock also leads to interesting deckbuilding decisions: Do you play cantrips/Environmental sciences if you know your opponent is on a mill plan?
For the grindclocker getting it down on turn 2, there's another interesting decision: Do you go for 2 charges to accelerate the mill plan and win outright, or do you start milling at 1? Each milled card is a card you know ISN'T in your opponents hand, so is getting that info a turn earlier worth winning by decking a turn later? Feels like it could go either way depending on the matchup. The opponent also gets to learn what card gets milled, you could dig them closer to their outs! Definitely plays nicely against blue scry, but blue is probably one of the colors that wants to be milling anyway. Turn 2 Grindstone turn 3 Spy Network seems like it could be a really fun, fateseal-esque play.
I think grindclock might lead to interesting play but it requires more testing. I ALSO really like Feldon's Cane though. It encourages a long game, but instead of milling, the Cane-player gets to win to their opponent decking. Here, the opponent isn't denied cards and so they get to cycle through their whole library, but the Cane allows you to re-buy your library and any creatures that died or spells you already used. I really like that second play pattern of knowing you have a powerful out in your graveyard and trying to get it back. It's also a lot cheaper to land at one mana. Maybe there's a world where both can see play, I need to get my hands on a Feldon's Cane first and see how it goes, and test Grindclock a little more to see if it's worth the trouble.
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3.) Fountain of Renewal: I really like Fountain of Renewal, but don't have one. Instead we've got another fountain: Radiant Fountain. This is a pretty contentious card; as a land, there's a real major deckbuilding cost to playing it, but gaining 2 life with a land drop is nothing to sneeze at. That's enough to survive almost any turn 2 on the draw (the only one I can think of is T1 Wildwood Tracker, T2 Duskshell Crawler). Fountain of Renewal is unplayable against aggro on the draw. That said, I'm very cautious about not letting aggro overtake the meta, but I can't tell if I'm TOO cautious. If so, maybe swapping the fountains would be a good play.
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4.) Sentinel Dispatch: I love the idea of this card so much as a defensive play with real draft implications. I've definitely had test drafts where one side scrapped for playables, this really isn't a free card to grab. But it's suuuuch a good defensive card without making an offensive deck playing it oppressive. I love it and I want it.
I have a tough time with this slot. Originally I had Sleeper Dart, which just sucks in my experience. I've considered Funeral Longboat, the 2 mana 3/3 vehicle with vigilance and crew 1. I wanna test the Longboat at some point, but right now I'm testing Ornithopter of Paradise. It seems like a bunch of archetypes might like the robobird. It fixes, it ramps from 2->4, it holds counters, and it blocks 1/1s. It has the potential to be a threat but can't kill until turn 3, and blocking a 1/1 leaves it open to the 1-damage removal. Pip heavy decks like black or decks running Radiant Fountain might like it.
I like it. It's colorless, it's a bit pricey, but it can do a little bit of everything. We'll see how it goes.