For whatever reasons I find these types of collaborative exercises very compelling. Here are some ideas that will evolve Tzigla more towards my ideal concept of collabs/challenges, which is hopefully shared by the majority.
Ranking/Rating. There's an issue with all art collabs that Tzigla could tackle, if it wanted to (Tzigla is now it's own entity apparently. . . I dunno). To illustrate it, think of boxing. Why are the light, middle and heavyweight classes in place? To prevent this:

Pitting Mike Tyson against mini-me might be amusing in the NES's Punch-Out game, but in real life it would simply not be fair. That's why the classes are there. In Taekwondo, does a white belt spar against a black belt?
Similarly, when artists combine forces and create collaborative imagery, such as with Tzigla, is it not equally awkward when your 8 year old brother makes a tile juxtaposed to a more experienced/skilled artist? Both tiles end up looking bad; your little brother's tile is put to shame by the rembrandt wanna-be and rembrandt's tile looks totally out of place, almost comical.
I'm hesitant to bring this to light, but it's a thought in most of our minds anyway, I'm just voicing it.
So, ranking/rating now. What am I getting at? Give people something to compete for and at the same time compel all involved to put forth max effort - Allow rating of tiles, maybe 1-10 stars possible. With this in place, you can now host invitation only collabs. Quite possibly only allowing artists with an average rating of 7-10, for heavyweights. Or hold a newbie competition for those maybe not as skilled, rating requirement 0-4.
Tiles could rated on multiple points - seamlessness, concept, how well the adjacent tiles were continued, etc.
What happens when someone logs in with multiple accounts? I could use my Pixelation and Twitter (which I don't have) account to contribute tiles to the same collab. Now one person is viewed as two, this is misleading.
Awards. For artists per collab: Highest rated artist on collab, artist with most tiles contributed, etc. This probably runs against the current Tzigla log-in mechanic, probably requiring a native Tzigla account. One person having multiple access accounts facilitates abuse too much.
Team Collabs. Two team captains pick their teams of artists, and these two teams compete against eachother, trying to get the highest user-rating once it's all done. Who does the next tile is decided by each team's rotation; all members take turns in a set order.
This is more in the spirit of digital art duels. Something I miss. Things like this could really boost the userbase because now it's not just a casual thing; kinda adds a layer to things. People like to compete.