The most important thing would be a really good extension system - the wordpress plugin system is so massively broken and there are so many bad plugins - this crap must be sorted out - but it is a good place to study how to produce bad code.
Extensibility is the key factor for any modern web framework / language / system.
Also Wordpress is last-century with deployment options - there are no (built in) options for deployment and a smooth upgrade process including backups and optional rollbacks. The "automatic updates" claim by WP is btw. a good example for the limiting view of marketing jargon and how technical terms can be abused and this way educating millions of flies to eat tthe wrong food. Upgrade without rollback of course does not make sense and is no way "automatic".
Please, if you build a new system, study modern web frameworks before and please do NOT use PHP.