We certainly don't want the combat to be non-interactive, which is something I was thinking about on my drive to campus. Giving the other colors their own types of evasion is nice, but if everyone's evading each other there's no exchange.
I don't really like it, but stretching reach into other colors is one solution. We need to come up with more flavor for white other than that it's all enchantments, so that we can decide how it would best deal with fliers. I think a normal white solution might be some sort of archer.

Here are two possible "mini-solutions" for white. They need to be common so they come up often enough, and so they must be simple.
Mark for Combat
Enchantment- Aura
Enchant creature
Enchanted creature is blockable.

The second solution is to put that ability on a creature.

Brightarrow Archer
Enchantment creature- Archer
­, : ~ deals 1 damage to target attacking creature. That creature is blockable this turn.

Finally, we could make a creature that can block regardless of unblockability. I could see this going in another color.

Since "unblockable" is acceptable and is used in the reminder text for both flying and landwalk, I thought perhaps "blockable" would be an acceptable term for concise wording.

It seems fine to me. Maybe blue can "jump".

Catapults :) more seriously, something which can give creatures flying, possibly at the expense of sacrificing the creature? Sounds red to me.

"target creature loses flying" (something like Gravity Well)

"target creature becomes blocked by ~"

"deal damage to flying creatures only"

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I think we should take care to let white have a good complement of ways to deal with attackers. The usual Neck Snap / Master Decoy / Pacifism / Crossbow Infantry / Condemn type effects. If we keep the density of them fairly high, and add a little reach as well, we don't need the rules-dubiousness of "blockable".

On the other hand, I do like "Target creature becomes blocked by ~". It's like a more interactive version of Trap Runner.

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Red more normally gets to burn nonflyers, but some effects like Rough // Tumble are probably plausible in red. If the invaders know anything at all about their target plane, then they'll have had a chance to prepare some burn spells that work better on flyers.

Red does also have a tradition of activated flying-then-sacrifice, on Stone Giant and Lowland Oaf. There's also things like Goblin Skycutter and Earthbind.

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I agree with Jack that red and black probably don't need more than one or two specific anti-flying cards, because they have removal anyway and that's usually pointed at evasive or otherwise scary opposing creatures.

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The mention of Trap Runner reminded me of Flash Foliage, which is a fun green solution to unblockable X/1s. Which in turn makes me remember Aetherplasm, and similar cards like Metathran Aerostat. So perhaps a blue solution is to put creatures onto the battlefield blocking target creature. It's not quite been done in that way before, but it feels like it could be convincingly blue.

I love the flavor of the Stone Giant ability. It's quite fun.
Anyway, "blockable" doesn't really work, but white could still do the same effect with better wording. Green, as always, can have reach.

I agree that "blockable" is a nice way to put it, but we probably won't end up able to use that wording, and that we may use the concept, but probably not.

An ability to put a creature from the battlefield blocking target creature makes sense, as does coming into play blocking, although to me the ambush flavour feels less blue; I'd expect blue to remove the creature from combat somehow, rather than leaping out and killing it. (Although not necessarily, blue does have a long synergy with 1/X creatures who are good at blocking.)

May I suggest Provoke? Or at the very least "This creature must be blocked"? If the monocolor tribes can't block the multicolor tribe, maybe they can force the multicolor tribe to block them. Also, we were talking about grappling hooks and what not before... this feels like grappling hooks.

Provoke is a nice idea. I did always enjoy that mechanic. It could work in any colour but blue, really.

I'm not entirely sure we want to use one of our mechanic slots on provoke, but it's a good mechanic that could fit the set's needs.

I think we can get away with not keywording it, if that's a problem. There are a lot of ways to spell out "must be blocked". White, Green and Red all have individual variations... we may want three different ways to do the same thing.

I looked in the current skeleton for common answers to fliers, and we certainly seem well supplied:

• White has five! Soulcapture Lance and Cage of Hands answer any creature, Blinding Chanter is a strong tapper for anything that's not monowhite (or colourless), Runearrow Archer works on all evasive creatures and Folded Light has reach. If anything that may be too much. Maybe Runearrow Archer should move up to uncommon where it kinda wants to be.
• Blue has Not a Drop to Drink as soft removal, plus its two counterspells Mystic Levee and Erode.
• Black has Turn to Stone as soft removal, Doomseek and Square the Circle as hard removal, and Chained Harpy with reach (though it's very small).
• Red has three common burn spells: Shock Variant, Violent Zephyr and Seismic Strike. That's slightly higher than most sets. Plus I feel against a dedicated mono-red deck the gold player will have to be holding some things back to block.
• Green has Zygotal Chain with reach, Cultural Lean as a weird Pacifism, and that's it. No Leaf Arrow or Plummet or Matsu-Tribe Sniper at common. I guess it'll have a Windstorm or similar at uncommon though.

It's true, we have plenty of answers.

• White could do without Runearrow Archer at common. It's definitely at least uncommon complexity.
• We started this set two years ago. Wizard's (visible) design philosophy has changed in that time. Doomseek would now be uncommon because that makes limited "better."
• Is three common burn spells a lot? Theros had four and Return to Ravnica had three plus one that only hit players.
• I think Green needs its normal flying hate spell, since it's lacking one. It's bizarre that this set would leave that out.

Re point #3, I think we want that many total, but something like 2-3 in red and 1-2 in multicolour?

I agree with the other three points.

Huh, okay, my instincts about common burn counts are just wrong. In which case the three common burn spells are just fine :) Recall Return to Ravnica also had Auger Spree, which is pretty comparable to our gold Cruelty from Above.

If we want to follow modern standards then Doomseek needs to either become uncommon or gain 2-3 mana in the cost.

I like Doomseek too much to mess with it, so I think we should brainstorm alternate removal for common and make that uncommon.