With socket AM4 you will have room for upgrades all the way to a 3950x with 16 cores and 32 threads. A future socket like socket AM5 on AMD would bring minimal benefits like DDR5 support, pci-e 4.0 (already supported in AM4), maybe you'd have usb 4 (i doubt with thunderbolt support because Intel)...
Basically only thing I'd care about is ddr5 support but considering you can still buy DDR3 and you have DDR4 memory available in 16 and 32 GB sticks, I don't see it as a feature worth upgrading to a new socket.
I think AMD's gonna have another series of processors on socket AM4, or at least the memory controller will support both DDR5 and DDR4 and a few select processors will also be available on socket AM4.
The APUs like Ryzen 3200g are still based on Zen+ (Ryzen 2xxx gen), so I would guess most likely there's gonna be an APU based on Zen2 eventually (probably using chips similar to the ones used in laptops)
You'll be fine with a socket B450 motherboard, good ones are available from around 70$
Cheapest I'd suggest is Gigabyte B450M DS3H : https://pcpartpicker.com/product/hpRzK8/gigabyte-b450m-ds3h-micro-atx-am4-motherboard-b450m-ds3h
It's better than ASRock B450M-HDV R4.0 or MSI B450M PRO-M2 [MAX] as in better VRM, better support for more power hungry processors like 10 core or higher... the two I mentioned I would not recommend for more than 8 core processors.
A very good board for the money would be Asrock B450 Pro4 at 75$: https://pcpartpicker.com/product/dQgzK8/asrock-b450m-pro4-micro-atx-am4-motherboard-b450m-pro4 followed by the ATX version https://pcpartpicker.com/product/PCKcCJ/asrock-b450-pro4-atx-am4-motherboard-b450-pro4
Then you have at 85$ Asrock b450m steel legend and gigabyte b450 aorus ,
https://pcpartpicker.com/product/hNdxFT/asrock-b450m-steel-legend-micro-atx-am4-motherboard-b450m-steel-legend
https://pcpartpicker.com/product/WcjJ7P/gigabyte-b450-aorus-m-micro-atx-am4-motherboard-b450-aorus-m
For highest future upgrade potential, msi b450-a pro or msi b450m gaming plus [max] or msi b450 gaming plus or msi b450 tomohawk would work (the versions with MAX in name just have a bigger size bios chip and fancier graphics in bios)
https://pcpartpicker.com/product/9C97YJ/msi-b450m-gaming-plus-micro-atx-am4-motherboard-b450m-gaming-plus
https://pcpartpicker.com/product/XJbCmG/msi-b450-a-pro-max-atx-am4-motherboard-b450-a-pro-max
https://pcpartpicker.com/product/pBWfrH/msi-b450-a-pro-atx-am4-motherboard-b450-a-pro
https://pcpartpicker.com/product/BHBhP6/msi-b450-gaming-plus-max-atx-am4-motherboard-b450-gaming-plus-max
https://pcpartpicker.com/product/jcYQzy/msi-b450-tomahawk-max-atx-am4-motherboard-b450-tomahawk-max
https://pcpartpicker.com/product/Hy97YJ/msi-b450-tomahawk-atx-am4-motherboard-b450-tomahawk
