I wouldn't be too worried about ZUN running out of ideas. Game makers regularly borrow elements from other games. The list you come up with here are aspects that he may or may not have borrowed from these other games, but Touhou still has a very distinct gameplay, storyline and artwork.
I'm not complaining about ZUN using elements from other games, in fact I like how he has, in a degree, touhou-fied the gameplay from the games I've mentioned above (but
not limited to them ), except that there's still often emphasis on grazing ( and occasionally, point-blank grazing, which is.. irritating, to me ), and no obvious way to know where your playthroughs can be improved, score-wise, other than trial-and-error or watching replays from other players.
I can't help but think that Touhou often references/homages other shmups/series.. when I look at
this for example.. and since grazing apparently has come to stay in this series, at least ZUN could make it more flexible or fun to do (for example, discouraging point-blank grazing as it can make scoreplay really inflexible, as in, "do it perfectly or else you'll lose too many points to even bother keep playing", and not having the bullet you just grazed be invalidated for further grazes; that way, you could still get better rewards for grazing masses of other bullets while being able to keep a grazing chain with a single bullet/stream, if there was such a grazing chain mechanism)
Anyways, if the next Touhou is a "traditional" Touhou shmup, I'd be really happy if the stages and boss fights can be at least as fun as playing Phantasm Romance 2.5 Plus. (of course, the "maze stages" would be highly unlikely to appear.. but the pacing was well done on that game, as well as the need to move around the screen
plus, I still wanna reach its 4th Extra Stage dammit)
