It was announced today that Granrodeo will be a special guest at Flow’s 15th anniversary show at the Budokan next January (1/30/2019).
Tickets have been on sale for months, so chances are running out. For details on the last lottery and the general sale, see the Granrodeo official site.
A new RG friend found this and gave it to me (thank you so much C-san!). The article apparently mentions Kiiyan overcoming his fear of flying. His tweet about it was more like “yeah, we’ll see….” It also describes him as being a popular seiyuu and mentions Shingeki no Kyojin.