Many things have changed and evolved from 2003. Many things are done now simply because "that's the way they've always been done".
Would you change languages? Architectures? Would you adopt a framework or avoid them because they go obsolete so quickly? Microservices? Serverless? Something weirder?
I have my own thoughts, but I'll add them later if anyone is interested.
CRUD from the get go. Man, using normal post tables was frustrating…
You couldn’t change the core philosophy in a major way without no longer describing wordpress. If I say adopt Laravel, I’m just describing OctoberCMS or whatever it’s called. If I want to use twig out of the box and ACF like data management in core, or focus on OOP I’m basically describing CraftCMS.
The things that make WP what it is, the core components that are non-negotiable to still be WP include (IMO) functional programming, PHP without a framework dependency, an emphasis on long term support, a relatively easy and open theming and plugin system, and the action/filter hook system. And the license, of course. Everything else is fair game.
Personally, somewhere between version 3 and 4 I would have completely reworked the database structure for new installs. That’s about the time it started to transition from a “blogging platform” to a CMS. I’d would also have liked to have seen, probably at v5, an slight change on version management. I’d like to see major versions be optional and feature breaking, with long term security support. Meaning minor versions continue to get patches independent of major versions. You can still provide long term support for older installs, prevent introducing bugs on all the old sites with features they weren’t built to use, *and* provide a cleaner cms for new installs.
Load and unload function to add and remove features natively without plugins. Don’t want WP comments, unload with a tick box, on the front end, no CSS or code for it.