Domain, Sunburst and Converge were all absent in MH3. Is it because the colorless theme of the set made difficult to make 5-color mechanics work well?

I assume that was a big part of it.

Any chance we will see Sunburst as a set mechanic again?

There’s a chance.

Will we ever see a land that produces mana that counts as 2 (or more) colors for Sunburst and the like (e.g. Adds a single W that counts as both W and R mana for Prismatic Ending)

I’m not sure the confusion is worth the design space. The former is big and the latter is small.

I feel undergrowth is a bit different from mechanics taken from individual cards and turned into a word (such as landfall, sunburst or kicker) because it's been widely-used and in particular is a big part of how green uses the graveyard. Innistrad alone had at least four 'undergrowth' cards: Spider Spawning, Gnaw to the Bone, Boneyard Wurm and Grim Flowering, as many cards as some named mechanics. It's been used with multiple other effects. In no sense is it a new or poorly-explored mechanic.

Being a focused mechanical theme is a different animal than having a few individual cards. The game is celebrating its 25th anniversary and we’ve printed over 18,000 unique cards. Finding mechanics that we’ve never done before on any cards should be seen as a cool thing that occasionally happens rather than something to be constantly expected.

Why was Crystalline Crawler given Converge over Sunburst?

It was set on Zendikar.

I've read your answer about Arcbound Wanderer, and I thought: are we ever getting a card with graft-sunburst? Or fabricate-sunburst?

The best chance for Mix & Match cards would be a supplemental product.

Why doesn't Corrupted Crossroads only make colored mana for colorless spells, rather than Devoid spells? The effect would be the same except for cute things like Sunburst, but it would be more backwards compatible and less parasitic, and read more elegantly. Any specific reason it didn't work?

Spelling it out makes it clearer and causes less confusion.

Why did you create Converge rather than simply reprint Sunburst?

Because they don’t work exactly the same.

Un-rules question: Do different colored Avatar of Me's need to be on the field for you to generate the color that they are, for effects such as converge and sunburst? Or are the colors allowable as long as you're playing by un-rules? Or maybe even that the colors are allowable if someone has that eye color?

Just because permanents exist of a color doesn’t mean mana can be generated of that color. Currently, even in silver-bordered world, you can only produce mana of five colors (White, Blue, Black, Red & Green). Remember that colorless is not a color.

Do you plan to bring back old mechanics by just giving them new names like you did with converge/sunburst?

The two mechanics are similar but not the same. Sunburst only went on permanents and always added counters.