The R5-3600 (6c/12T) and 3700X (8C/16T) are both pretty great values for levels of performance given their very reasonable price points....(Few would argue yet that the 3700X or above really help 'all that much' in gaming yet over the 3600, if gaming is a concern, but, if you are content creator, etc., perhaps the 3900X's 12 cores/24 threads might be of interest.)
If you even think you might again need (want?) a CPU upgrade in a years time, I'd probably look at an X570 mainboard, but, B450 with latest BIOS offers a great value currently, as PCI-e 4.0 does not affect/impact much currently, beyond some very impressive CrystalDiskMark sequential reads/writes scores...
No telling whether or not Intel's upcoming socket 1200 will last beyond the duration of 2020, as far as 'upgradeability' (although it would indeed be hard to imagine 'needing' some sort of upgrade very soon after getting a new 10700K/10900K)