Short answer

Customize or change arc42 only if you absolutely need to. The standardized structure of arc42 has a high recognition value, and many people are accustomed to it.

Longer answer

Keep arc42 modifications to subsections and leave the high-level structure unchanged.

I (Gernot) encountered the following modifications during real-world projects:

• User-interface view (UI-design, UI-forms, common layout etc). Sometimes, UI details can be highly relevant for other architectural aspects, e.g. handling application state, integrating with workflow technology or similar.
• Data view, explaining technical data models or concrete table structures. Often relevant in very data-centric organizations, where portfolios of applications are built around these data structures.
• Developer guide, containing detailed development information, sometimes called development use cases.