Actually, Nevalistis did in fact state what the S18 rewards will be. Going forward, they will be mirrored to a season that is approximately three years apart from the season at the time. So season 19’s rewards will be the same as season 7, season 20’s rewards will be the same as season 8, etc.
It is possible that in addition to the recycling of the seasonal rewards (so everyone can eventually get them all), new items will be added from time to time as well. Those too will then be available twelve seasons afterward as well.