Must have? The ability to use Visual Studio Code + the remote server SSH extension. This effectively allows someone to do "local" dev on the remote server.
Why it matters:
1) Ease of spinning up a dev env, especially for larger (DB + media) sites. Every host has clone an env. Typically that's for staging. This is just added envs for "local" dev.
If that cloning-for-local-dev can automatically add things like wpcs, and PHPStan (or Psalm) and other such tools - with specific instructions how to update VSC that would be super helpful.
2) Privacy and security. This mitigates the devs' responsibility for any data on their local machine. Obviously privacy and security matter, even for WordPress ;) I've always been curious what GDPR's position is on such data. My hunch is, it's subject to the law. That is, for example, a request (read: click) to remove account should remove that data *everywhere*.
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I mention the above because not all the major WP hosts support it. Afaik, WP Egine does not; Not sure about Pntheon but it's very opinionated so maybe not?? There might be others.
p.s. This would be magical...some premium plugins are licensed per domain. That is, prod is a license, staging is another, dev envs more licenses. If there was a way to set up some sort of proxy so for certain plugins the pings to the license auth'ing all look like the same domain / IP that would be a cool - and helpful - trick.
I'm not sure that's even possible but I figured it was worth mentioning.
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Edit
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It looks like you support a staging env. That's good, tho' as expected :) What's missing - I presume - is support for additional "local dev" clones.