Would highly doubt there'd be any Alpha 3 playthroughs in the community streams as they'd want all new features to be revealed in the Dev streams. Could have a few who are attending Gamescom (the youtube/twitch types, not just a random gamer) be part of the live streams with Ed/Bo and co. and some who are streaming from their homes?
I'd think the first Dev Stream could be all of the new features in alpha 3 and then the following days could be going into more detail of those features and upcoming features they're working on. And since all of the new features would've been revealed before the first Community streams there wouldn't be any spoilers.
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In other words, the Dev Stream schedule could go something like this:
Day 1: New Features in Alpha 3
- Park Management
- Blueprints
- Water
- Lighting
- Recolorable scenery objects
- Fairy tale theme
- Etc
Day 2: Park Management / Blueprints
Day 3: Water/Lighting/Scenery Objects/Theme
Day 4: Upcoming Features / things they're working on
Edit:
At least I'm hoping that's what happens. My biggest fear is that the Community Streams will just be community videos from Alpha 2. Even though we've all seen them (sometimes more than once or twice), they might just want to show the crowd at Gamescom how creative you can be in the game.