Body Dropper Kathari Bomber
This cube is not big on tokens or sacrifice, so this moves the Multi-color on at least one of those angles.
Caldaia Strongarm Saddleback Lagac
Both provide two counters. Strongarm is a 5 drop, but it has the better body, and there is always the option to Blitz
Celebrity Fencer Trusty Retriever
This guy has 3 hits for the second ability, so he's just a 4mv 3/4
Celestial Regulator Dramatic Rescue
Both the current Azorius cards fulfil the same role. They can protect one of my creatures from removal, or reset an opponents creature to remove counters. Rescue might be the better card on the reset method (By forcing them to replay the creature) but it is harder to cast, so I'm going to pull it.
Echo Inspector Thunder Drake
Not wanting to go in on a 2 spells theme.
Psionic Snoop Naiad of Hidden Coves
So Naiad was part of a back-up spellslinger sub theme, but now we're getting more counters cards at common for Blue, which was the worst color.
Raffine's Informant Ainok Bond-Kin
Not sure I'm interested in Outlast, On the fence if this is a worthy swap, but I'm interrested in Connive.
Revel Ruiner Driver of the Dead
The range of 2mv cards to hit for driver is fairly small.
Security Bypass Befuddle
Pretty obvious this was a filler card.
White: Easily the richest color (at common) for +1/+1 counters along with green. part of the reason the cube took as long as it did. I think This color has the most room for tweaking.
Blue: This was actually very short for on theme cards at common. Was the exact opposite of white. I was glad to get a few cards out of NEO. It still has a few generic cards to round the count. I opted for primarily instants to pair with the two pyro grow creatures in red to make a link between the two.
Black - Black ventures into -1/-1 counters as a foil/ balance for the counter growth. There is also a small Unearth subtheme due to one of the WB cards.
Red - Red has a lot of deal damage get counter cards, but I didn't want to get so aggro it imbalanced the colors. I linked it with U a bit thanks to Pyre Hound and Pyroceratops. I also made sure to include a few more 'big' burn effects like 4 to creature because shocks will not do well against a counters theme.
Green - Green needed no assistance to be a strong +1/+1 counters color. I opted for only one of the 3 (I think) trample for +1/+1 counters creatures. I didn't want green to be super unblockable. I was also glad to see that a lot of fight effects relate to counters as well.
Multicolor - I didn't want to be imbalanced on color count, but some colors really lacked counters cards at common (Simic cards for counters were apparently largely uncommon+) This is where I got into the subtheming. Breathless Knight encouraged some Unearth in WB. Dramatic Rescue and Turn to Mist created a bit of a blink/bounce theme, which is nice to also reset opposing creatures or save your own. The UR ones lean into the spells theme.
Lands - Simple fixing, two tapland cycles. I opted for the Gainlands and Snow basics. They were recent, so I was happy to use them Also with Gainlands in NEO the art is fantastic. I did consider mana rocks of some kind, but it would eat a lot of space in a very small cube. Considerations: Diamonds cycle or Borderposts. Why even consider rocks at that point? Might be right on that one.