Vintage Cube is thriving on MTGO, including via community events. Arena has a similar history of community events, so why not allow Vintage Cube to happen here? I think that a large portion of the potential upside could be captured with surprisingly little work.

Specifically:

1) Make implemented-but-not-craftable cards craftable-but-only-legal-in-direct-challenge
This is the only thing I realistically expect might happen as a result of this feature request, because it is a low-cost way to unlock marginal revenue and discover whether a market exists for the rest of this feature. (Note that Arena already warns when cards are not legal in certain formats on the crafting screen; for example, try to craft Zidane Tribal.)
As you know, specifically the following cards are implemented, not yet craftable, and of interest for Vintage Cube:
- the Power Nine
- Sol Ring
- The ten Alpha dual lands
- Mox Opal
- Ponder
- Preordain
- Serum Visions
- Thought Scour
- the ten Ravnica signets (because talismans are not implemented)
- Iona, Shield of Emeria (merely nice to have, but take my money!)
- Unmarked Grave (merely nice to have unless archon of cruelty is printed)

By itself, this will at least make it possible for the community to attempt to find a way to replicate the Vintage Cube experience using the cards available on Arena, which will only increase as bonus sheets and anthologies continue to accumulate. If additional work is at all on the table, read on.

2) Print single cards that should parse cleanly (i.e., use already-implemented functionality) and are significant archetype enablers.
I think these are high bang-for-buck cards. (The Forces are sorely missing from Timeless as well; perhaps a hypothetical future Timeless Anthology could kill two birds with one stone?)

• Tolarian Academy
• Force of Will / Force of Negation (compare Force of Vigor and Commandeer)
• Tinker
• Entomb
• Vampiric Tutor / Imperial Seal
• Flash (admittedly not certain this parses cleanly)
• Mishra’s Workshop (“spend this mana only” is on over 90 cards on arena)
• Brain Freeze (not asking for Lion’s Eye Diamond)

3) Print single cards that should parse cleanly and are among the best in the cube

• Mana Crypt
• Mana Vault
• Karakas
• Library of Alexandria
• Strip Mine
• Skullclamp

4) print single cards that should parse cleanly and are iconic or combo enablers

• Grim Monolith
• Basalt Monolith
• Triplicate Titan (Tinker/Flash overlap)

5) Print specifically Gut, True Soul Zealot

It’s in the top 50-100 cards, should parse cleanly, and the art is already on Arena as Gut, Fanatical Priestess. Just ignore “Choose a Background” to avoid implementing that.

6) Wishlist: print alternatives to Animate Dead and Necromancy that are easy for you to implement

I understand that asking for actual Animate Dead and Necromancy is unrealistic, but reanimator is a central archetype. I understand that WoTC employees can’t look at custom card designs, so I won’t make suggestions, but it seems likely that alternatives could be designed and implemented easily (for example, using digital-only mechanics) if you wanted to surprise and delight customers.

7) Entirely unrealistic wishlist: implement broadside bombardiers

I understand that random commander set cards, even if they don’t require new mechanics, are unlikely to be implemented for this use case.

8) Entirely unrealistic wishlist: implement the monarch cards that see play in cube

See above.