I'm going through the process of bringing the cube down to 375 (this is about what my box fits comfortably). It ballooned to 391 while I was messing with additions, so this has turned out to be a bit iterative.
Goal:
40 - with 4 land for each pair
40 - with 4 multi-colored for each pair

260 - with 52 of each color
35 - Artifact and generic lands
-or-
255 - with 51 of each color
40 - Artifact and generic lands

I'm mainly looking at artifacts and generic lands for cuts. This might be hard since that means 15 cuts just from that column. I am also considering 51 in each color, which would make the artifact cuts more gentle.

I'm currently looking at lots of changes to give GW and GR better identities (enchantments and beatdown, respectively). Also, adding a little bit of planeswalker support. Removing Energy (sorry Aethersphere Harvester) is something I'd like to do, just because it feels like it could be a trap for inexperienced drafters. Similarly, removing Monarch because I'm not in love with that mechanic in 1v1 games.

Land changes:
Azorius - Shock, Pathway, Bicycle, Manland
Dimir - Shock, Pathway, Bicycle, Manland
Rakdos - Shock, Pathway, Bicycle, Painland
Gruul - Shock, Pathway, Grove of the Burnwillows, Fetch
Selesnya - Shock, Pathway, Horizon, Fetch
Orzhov - Shock, Pathway, Horizon, Manland
Golgari - Shock, Pathway, Horizon, Fetch
Simic - Shock, Pathway, Horizon, Fetch
Izzet - Shock, Pathway, Horizon, Manland
Boros - Shock, Pathway, Horizon, Painland

Other lands:
City of Brass, Mana Confluence, Ancient Tomb, Prismatic Vista, Ash Barrens, Mishra's Factory, Terramorphic Expanse, Evolving Wilds

Green with fetches making it the color you want to be for 5-color.

Archetype adjustments
UB Tempo/Ninjas
GW +1 Counters
GR still looking for an identity