The EU already has previously published an initiative, which was voluntary but which many phone manufacturers did end up getting on board with, regarding using micro USB connectors for charging: https://en.wikipedia.org/wiki/Common_external_power_supply

I think that with USB power delivery and USB type C connectors, we could see a similar move again (or an update?) from the EU or another larger government (China?).

Getting user replaceable batteries, or at least more phones with this option even if it's not mandated by regulation, may come about from some of the "right to repair" legislation which is starting to take shape in the USA. I'm sure the EU has a similar (existing?) set of regulations in the works. I can't see a mandate of everyone having to use the same exact battery or tool-less requirements but I could see a mandate happening that a battery has to be able to be replaceable by a 3rd party service center and the OEM must sell battery replacement kits and instructions for a reasonable cost to 3rd parties for X years after a phone goes on the market. This wouldn't kill Apple, Apple is already replacing iPhone batteries with Apple provided parts using Apple technicians for only $29 in 2018: https://support.apple.com/iphone/repair/battery-power