There is no such thing as a Mac mini 2020. It's a Mac mini 2018 with more baseline storage. That's the only difference.
Since Apple recently updated the Mac mini with the baseline storage increase and passed it off as "new" [sic], there is zero chance of a new Mac mini at WWDC.
Remember, >85% of Macs sold are notebook computers and most of the desktops sold are iMacs. That probably means Mac mini comprises about 1-2% of Mac unit sales. Since Macs are about 10% of Apple's revenue, we are talking about less than 0.5% of Apple's total revenue from the Mac mini product line. It's not a relatively important product.
Apple took four years to update the 2014 model to the current 2018 version. In all likelihood, it will be another couple of years before we see a mini with upgraded performance.
If (and that's a BIG if) ARM comes to the Mac family, it would debut in a notebook computer, probably an entry level one to fill the space of the departed 12" MacBook 2017, for a target audience that does not rely on the ability to run x86-only software.
The Mac mini is powered by AC from your wall. The power-saving benefits of ARM are less valuable for a desktop computer.
Remember that ARM in Mac is a rumor not a guaranteed future given. Frankly, the way Apple is pushing iPad Pro toward a more computer-like experience makes me believe that ARM MacBooks are a fairytale.
One thing for sure, if an ARM Mac does debut, it will be with a smaller complement of compatible applications than what runs on an Intel Mac today.
If you want a Mac mini now just buy a refurbished unit.