Elgg 3.0 will indeed include some schema changes. They are related to

• denormalizing metadata/annotations (the table metastring will dissapear)
• site_guid column will dissapear for entities
• config and datalist tables will be merged
• removal of entity type specific tables (not yet finished)

Elgg will provide migration scripts to move all the data around. If you have no multisite installation, no customizations on the tables and are using the Elgg API for accessing the data, it should cause not that many problems. Nonetheless going from Elgg 2.x to 3.0 will be an update that requires attention.

Schema diff 2.3 to master.

Any reason to maintain an entire fork of Elgg? Perhaps it would be more of an incentive for your contributors to contribute to Elgg directly if it weren't?

Yes and we want to move in that direction.  Version 2, Composer, ..  Thanks!  But for now, participants will be able to earn point for contributing to Elgg!

Elgg 2 comes with:

1. Improved simplecache. Static resources (css, js, image files) can exist anywhere in the view system. When simplecache views are generated their location is mapped against the view system location. This allows you to serve files directly from the disk bypassing the PHP engine. You need to configure your serve to allow symlinks and to symlink the cache directory to simplecache directory. http://learn.elgg.org/en/2.3/admin/performance.html?highlight=symlink#performance

2. Boot time cache caches a bunch of run time data (e.g. plugin settings) to shorten boot time. You may want to look into TTL setting to find the right value for your site.

3. Files are served using a special handler, which serves the file without booting the engine. You can achieve better caching by sacrificing file access controls and serving all files without binding them to user sessions.

You can fork the starter project and add your dependencies there. Whenever you need to uodate the dependencies, you run composer update locally, commit the lock file and pull on production, and run composer install (not update) on production. That way your dependcies stay in sync.