The Most probable in my opinion might be either another Fighting Game or an Action-Platformer ala Sunken Fossil World.
The 20th game in the series (or 20th year, game, etc. in anything ever) is often special with no explanation, so ZUN is probably going to try and one-up himself like he tried in the past. So with ZUN spending effort on that, the other option is Twilight Frontier (TasoFro), I have slight doubts that they would continue at their current pace given the infamous Development Hell-adjacent that was Sunken Fossil World, but they might decide to just continue marching on and if they do then I'm going to be worried for their health.
This is all assuming that we get a 19.5 game spin-off in the first place. They could skip it and this becomes the "Touhou 20.5 Theorisation" Thread. But more chances to see novel takes on bullet dodging probably keeps those folks alive. I know it keeps me alive.