1. Boundaries beat buzzwords
I split the codebase into domains with explicit inputs and outputs: persistence, integrations, REST, admin UI, background jobs. WordPress hooks sit at those edges. If a WooCommerce sync class reaches into HTTP controllers directly, you will pay for that shortcut in every future release. Small modules with injected dependencies (constructors or a tiny container) keep tests possible and coupling visible.
2. Data is the longest-lived API
Options keys, custom tables, and post meta outlive class names. I version schemas, document what must never be renamed without migration, and avoid stuffing serialized blobs that no one can diff. Long-term projects need export and import stories—even if v1 is a SQL dump plus a README.
3. Public extension points are a commitment
Every do_action / apply_filters I add is a backward-compatibility contract. I prefer fewer, well-named hooks with stable parameters over dozens of “utility” filters nobody can grep. Deprecate with notices and semver discipline; agencies build on these seams.
4. Upgrades are features
Activation hooks are not enough. I ship a migration runner keyed off a stored schema version, idempotent steps, and logging on failure. Rollback may be manual, but “silent half-migration” is unacceptable on production databases.
5. Operational shape
Long-term means someone on call. I design queues, retries, and feature flags so risky work can be toggled without redeploying PHP by hand. Health checks and admin status screens beat Slack archaeology when a cron queue stalls.
6. Onboarding is architectural
A diagram in the repo and a CONTRIBUTING section that names the request lifecycle matter as much as code style. If a mid-level hire cannot find where REST auth is enforced in thirty minutes, the architecture failed—regardless of patterns folder depth.
WordPress plugin architecture for long-term projects is constraint design: what you refuse to couple, what you promise integrators, and how data survives your next big idea. Everything else is implementation detail.