1. Structure like a product, not a snippet
Use PSR-4 autoloading (Composer), namespaces, and folders for Admin, REST, Services, and Integrations. The main plugin file should bootstrap and register hooks; long procedural scripts become impossible to review at scale. Inject dependencies where practical (constructor or a tiny container) so services can be swapped or mocked.
2. Own your data model early
Post meta and CPTs are fine until query volume and reporting hurt you. High-cardinality or relational data deserves custom tables with versioned migrations (dbDelta or a migration runner), explicit indexes, and cleanup on uninstall if policy allows. Document whether rows are per-site or global in multisite.
3. Expose extension points on purpose
Besides using core hooks, emit prefixed action and filter names at stable boundaries (after validation, before remote calls). That lets agencies add behavior without forking. Keep hook signatures stable; when you must break them, deprecate with notices in debug mode and semver-major releases.
4. Tests and CI are part of scalability
Add PHPUnit for pure PHP services (with Brain Monkey or similar where helpful) and integration tests against the WordPress test suite for critical flows. Run them in CI on the minimum supported PHP and WP versions you advertise. Scalability includes “we can refactor without fear.”
5. Multisite, i18n, and caching
If any client might run multisite, decide up front: network-activated vs per-blog settings, and where options live. Load a text domain correctly and avoid concatenating translatable strings. Assume object cache exists: cache expensive option reads and remote metadata with clear TTLs and invalidation hooks—never silent stale data in admin.
6. Performance is a feature
Batch queries in list tables; defer heavy work to Action Scheduler or cron; avoid autoloading huge option blobs. Profile admin screens the way you profile the front end—editors feel slowness before visitors do.
Scalable WordPress plugin development is mostly boring discipline: structure, data ownership, tests, and explicit contracts. Nail those and the plugin can grow with the business instead of blocking it.