MORE to it than just the type of connector
Standardizing the connector is not sufficient to make chargers universal.
Power negotiation protocol must be standardized. AND
Power output capacity must be standardized.
(Or at least a minimum power output level must be established, and that minimum should be realistic for modern-day smart phones that provide fast recharging times!)
USB-PD (Power Delivery) may be a reasonable standard to adopt. It is currently at version 3.0
But even that has options, and extensions. AND, the extensions are currently in use (by some high profile smart phones).
The high-end Samsung charger uses the PPS (Programmable Power Supply) extension to PD. It delivers 45watts to the phone, using a custom voltage.
AND, its output current is high enough it should require the 5A-capable power cord (another extension to the USB-PD standard).
[See https://www.gsmarena.com/dont_just_buy_any_45w_pd_charger_for_the_samsung_galaxy_note10-news-38837.php ]
To accommodate all of today's phones (allowing a single charger to optimally charge all phones), we'd need (at least) a 45W USB-PD PPS with 5A cable.
Historically that might be expensive. But all the power supply manufacturers are now touting their new GaN (Gallium Nitride) based electronics. GaN is cited as enabling smaller physical size of the charger while also providing enhanced efficiency and reliability. AND, they cost less than previous designs.
[See https://www.theverge.com/circuitbreaker/2019/11/11/20959970/hyperjuice-hyper-100w-gan-wall-charger-gallium-nitride-kickstarter-preorder-usb-c-usb-a ]
So, maybe it _IS_ economically defensible for the EU to mandate a 45w USB-PD PPS 5A standard.
That covers all the phones we know about. TODAY.
How to we prepare for advances in battery technology?
It hasn't always been Li Ion. It used to be NiMH, and NiCad before that. One possible future is Lithium Sulphur.
Researchers in Australia have announced enhancements to Lithium Sulphur battery design that may open the door to higher capacity phone batteries.
[See https://www.monash.edu/news/articles/supercharging-tomorrow-australia-first-to-test-new-lithium-batteries ]
[See https://www.eetimes.com/a-new-lithium-sulfur-battery-with-an-ultra-high-capacity/ ]
To allow higher capacity batteries to be recharged in a timely manner, perhaps the EU standard should mandate that all chargers support the highest level of USB-PD "Power Rule", allowing 100W to be supplied (at 20v and 5A). This would provide some room for growth in the smart phone charging system, and ALSO allow the same charger to be used to power OTHER devices (tablet and notebook computers and even some laptops). THAT could be a really universal charger.