Santoshi is completely right. The reason why you basically only see NES, Genesis, and SNES clones is because its really the only systems the market can bear. TG-16 clone would be terrible idea (business-wise). Its not a very popular system outside of the retro gaming community and many of the games are really expensive. Japan is not a very big market when it comes to clones. I think their main targets are U.S., Europe, and Brazil. The only way I could possibly see it if they made an adapter for the R5, but even the costs to produce the adapter may not be worth their time. They aren't going to do anything like this until they feel the R5 is a hit. Whether its a hit depends on if it delivers. I think Hyperkin has a lot riding on this system. They are not a big company and this is a pretty big project to undertake.
Also for those suggesting disc-based clones. I highly doubt it will ever happen. Just too many issues.
P.S. I am not being a hater. I would love to see good clones made of all sorts of systems, but from the business side I completely understand why they don't. Most likely anything outside of really popular systems would be a disaster financially. I think people totally under estimate how much something like that costs up front to produce.