(Still a Modern Horizons 2 update to come! This one's just a lot simpler.)
I intend on removing all the allied colour basic landcycling cards - for Ancient Excavation, it'll be Barrin's Spite I think replacing it, whereas it's less clear for Treacherous Terrain. I actually like it a lot more - it's a clear RG finisher even if it's potentially a bit too strong for draft level play. Having a clear "This is going to deal everyone 8+ if the game goes too long" is more important than some of the other effects anyway and, frankly, it makes more sense for RG of all colour combinations to have additional cycling access. I'm just disliking that, frankly, colourless landcycling makes deck construction a little easy. Think about it - the Onslaught block cyclers only fix for the colour of the card, and the Alara block ones require that you already have a source of that colour. It feels balanced for "Oh the spell side is harder to cast" to also mean that the cycling part is easier, but the effects are also... usually a lot stronger? Four birds doesn't seem too big but white/blue LOVES that effect, it's honestly very strong. Reanimation with haste? Great. Exiling enchantments and artifacts in a format rife with them? Sold. Even "Draw your hand then discard that many" really downplays a lot of the sadness you can sometimes have with a 60 card deck in draft by going "Ok, filter for that perfect hand then fuel Arambi ANYWAY". It's just a bit too good, and it makes the enemy coloured section feel worse. Originally I added the third set of multicoloured uncommons so that allied would get cycling, while enemy would get <more substantial effects>. Now I would prefer "draft indicators x2, powerful card x1". I feel the replacements live up to this! Bruenor is a CLEAR Equipment Deck Lord, while Hurl Through Hell is all about giving Rakdos the removal it loves in this sort of environment - exile a threat, but you draw a copy for the next turn cycle. Even better if you exile a flash threat. ;-) Detention Sphere is a safety valve choice for now - I feel there are better choices but, if tokens are a concern, it's nice to have something that can do something about it. Sythis is the obvious GW enchantress payoff should the cube need it and, if the cube doesn't, it is easily cuttable.
Now, other choices:
Modern Horizons 2 has a LOT of cards I need to think about too, and will be updated soon, but this was an easier update for me to make. Adventures is a low powered environment, even if the Commander decks with this set launch gave a LOT more things to think about. Knowing from the start that Venture wasn't going to be used while Clay Golem is the only admission that dice rolling exists made the update easy - it was all about choosing cards that also ignored those concepts, helped by most of the other choices not being too good. Horizons is... wow, it's a defining set for a reason. There's creep and seep everywhere, very unique effects that need to be considered. And Innistrad spoilers are coming very soon? A LOT to be excited about in the cube world, for both this and my other three cubes. But this is my project that is the least set in stone currently so it's the one I wanted to update first.