assuming there is actually a market for a 350CC engine which is another great unknown.
A look at history says that it shouldn't be a market. Way back when there was 370/ 400 cc motocross engines fighting for the open class. well as it goes more is always better so bikes evolved into 490 and 500 cc monsters. At this time stadiums and man made jumps where becoming the hot ticket. Negotiating a cr 480 or a yz 490 around a stadium must be a hair raising event. Up until the point that there was no open class stadium events. Why would anyone want to risk eating it on these huge man made jumps when their lap times wheren't as fast as the 250's?
At this era nobody ever got the bright idea of sleeving down/destroking an open class bike, which should have been worlds cheaper and easier to do then making a 250 bigger. Scratch that, KTM and Maico where playing around with displacement options looking for what is big but not too big but they could not concievabley come up with the sales numbers of the "big four". With 2 of the big four dropping out of open mx bike sales altogether. Yamaha barely scraggling on with selling the old YZ as a WR and Honda and Kawasaki selling old designs with left overs from the last year's 250's improvements.
Noleen had a short run on trying to make a 360<?> out of the Yz250. Very few people though that it was advantageous to buy a brand new motorcycle and dump 1/2 as much more into the engine to run a different class against bigger bikes.
So what we have now is Honda discontinuing the cr 500 line, Kawasaki selling ancient 500's to the more is better crowd and European "cottage industries" selling the "little bit more than a 250 but not anything brutal"
Unfortunately for the US you are going to have to wait and see if two strokes will be allowed to be manufactured for sale, the KX500 discontinued, continued success of the European 300's and then somebody saying" hey we should be able to sell something bigger" before you would hit a 350/400cc target again.