As much as they intend it to be a one-off thing, who knows what money and a good attitude in the band will do.
The tour is shaping up to be a success, let's say it sells out everywhere and Kiske and Weikath don't want to kill each other, if two years from now promoters offer them more money for a second round, why would they refuse? where Helloween do even go from here, back to "just another" album with Deris? they've always been strong in Italy, they always played the same venue in Milan where Avantasia, Alice Cooper and Blind Guardian play (just to give a comparison), and for this concer they've been upgradted to the place Iron Maiden play when they have an indoor show.
Maybe if they do like Dio back with Iommi back in 2006 and do one step by one, they might feel like doing an album together. But of course 2019 is meant to be for Gamma Ray and maybe Unisonic...