Free flight 2D shooters: Bangai-Oh (N64, Dreamcast) probably counts. Armada (Dreamcast) might count, though it starts to mix into other genres. If you want to stretch even further, you might include games like Drox Operative (PC) which is almost Diablo with space ships (except not. Soldak makes games that on the surface are similar to Diablo, but underneath are more complicated and ‘alive’ than Diablo has ever tried to be.)
As for horizontal versus vertical 2D shooters, there are I believe two main factors to feel. One is the direction of travel (which can give a sense of speed), while the other is the orientation of the screen (which matters in relation to the direction of travel). That would theoretically give you four different types of “feel”, but I can’t think of any 2D shooters that used horizontal shooting on a vertical monitor. However, your mention of rail shooter stage interaction does remind me that it at least feels like side scrolling shooters have more interaction than vertical. Vertical shooters often really are ships flying over a background, while side scrollers are more likely to have things like ceilings and floors and to throw obstacles into your path.
As for 3D rail shooters versus 3D free flight, the big difference is that rail shooters can constantly have action. (You hit this around 13 minutes in, but I’d already written this part by then.) The only downtime on a rail shooter is intentionally placed. With free flight, you might have to fly around to even find enemies, and you will see a lot of empty space. You will also likely face a much larger number of targets in a rail shooter. This is true to a lesser degree with 2D shooters, though 2D has the potential to cheat a bit more with spawning enemies just offscreen. (There is a third kind of shooter that is neither rail nor really free flight. There is the arena shooter, or Geometry Wars style shooter. Some of those will dump tons of enemies onto players, more than even rail shooters. The player has free flight within the screen, but is often limited to a single screen. However, these games almost have to flood the player with enemies as there isn’t much variation in how enemies arrive or how the player can face them. The idea doesn’t translate well into 3D.)
As for chase view shooters and the idea of “too much space” and the difficulty of hitting enemies, when it comes at least to the scaled sprite-based games, the difficulty in hitting enemies to me tends to come from not being immediately certain whether I’m in line to hit an enemy. These games ask the player to aim in three dimensions without ample clues to judge positions. You get size and maybe a flickering shadow that is some distance from the enemy that casts it. This isn’t quite as bad when you move to polygonal graphics because at least there you have the angles of the polygons as an additional guide. (There may also be finer degrees of animation and movement as you get to more advanced hardware, making it easier to judge position from movement.)
As for the lack of rail shooters… They arguably didn’t work that well before sprite scaling, and weren’t necessarily great even with sprite scaling. They were more complicated than overhead and side view. Later, games could give players free flight almost as easily as they could rail shooting. 2D shooters grew into the whole bullet hell genre, which requires a precision that rail shooters have trouble delivering and flood the screen with an amount of content that I don’t believe rail shooters can handle.
*nods* Yeah that sounds about right. ESPECIALLY on free-flight 3D shooters. Those may actually be EASIER to make!
Space Marisar though is an interesting experiment in on-rails bullet hell. The STAGES aren’t but the bosses are…I mean of course it is. It’s Touhou Space Harrier. Takes a stage or two for the wackiness to happen though.
Still I wanna see more experimentation with the genre. The good news is that I appear to not be alone what with that Star Horizon on Steam Greenlight and I just downloaded a demo of a tunnel-ish (not sure if I wanna count these or not) game called Hyper Void on PS3 so maybe I’ll b getting my wish all of the sudden! XD *plays Hyper Void* Well…that was dry. Not Futuridium dry but…*shrug* well I got faith in Star Horizon.