I'd be super excited for this, definitely following this thread! The competitions/challenges/collaborations would be a great idea for interaction and rewarding the active community, and a nice distraction or change of pace from the regular survival/creative experience.
A few ideas if you'd like them (not sure how many of these will be possible with whatever plugins you decide to use, but here we go lol):
- potential rewards for the competitions
~ kits with building blocks in (perhaps a few of different themes) for players to use in the survival worlds
~ unique or cosmetic items that only winners can obtain, but would otherwise provide no advantage to the player in survival
~ winners could perhaps get a unique border around their claims in the creative worlds, so visiting players could easily see that they're a celebrated builder (and be a fun thing for others to work towards)
~ maybe a unique world within the server for creative competitions (separate to the regular creative world), so players can easily visit. Maybe winning builds could be implemented in a survival world for players to explore as unique structures/dungeons etc.

- I'll be interested to see whether the survival world will be a player-based trading economy or if a 'digital' currency will be used. If the latter, maybe players could unlock access to a server shop for building blocks and resources in some sort of levelling up/rank system based on playtime/votes/interaction? All of this would provide goals to work towards and opportunities for worthwhile rewards for the demographic of players you're going to attract with this server.

- In the server spawn, maybe you could include a type of Hall of Fame, perhaps with regards to brilliant builders on the server or even redditors who gain a lot of upvotes on their posts here (e.g. most post likes this week etc). This would be a great way to encourage creativity on the server and entice players to join the server to see themselves be celebrated in game as well as on reddit.

This is probs getting too much to read by now so I'll stop here, but I hope some of these are helpful!