If it's a workstation I would go for TR4 for sure on the AMD side. Theadripper will also be receiving a refresh and will use the same socket but as some have pointed out, there are no guarantees that it will work in current TR4 motherboards. It may be like Intel Core i series 8th gen and by physically compatible but not work. I wouldn't let it hold you back, it's the best workstation CPU on the AMD side and it does great in many workstation things like encoding.
I wouldn't expect any more than 64GB from a desktop or even enthusiast motherboard platform. If you need 128GB don't wait for a gaming motherboard maker to come out with 128GB board (whenever THAT happens) and pair it with a non workstation CPU. I've only built 2 PCs that had 128GB of RAM, one was a server and one was used for heavy simulations. Even if you could put 128GB of RAM in your typical gamer quad core the CPU wouldn't have anywhere near enough cores to utilize that 128GB of address space effectively.
Recommend not trying to start with X amount of RAM and then add more that's just asking for problems. Buy the RAM in one 128GB kit at the same time you are buying the motherboard/CPU. 8x16GB is the most common 128GB setup.
Also recommend not waiting for motherboard makers to do you any favors when it comes to adding new features that were not included in the board when you bought it.