Singe Player Shmups with a lot of content?

[dre1]

Are there any shoot-'em-ups with engaging single-player content—such as branching paths (like in *Darius*) or unlockables like ships—that make multiple playthroughs fun?

[PC Engine Fan X!]

Well, with the PC Engine Arcade CD-Rom2 stg of Ginga Fukei Densetsu Sapphire, you can choose from four different fightecrafts + plus it supports simultaneous two-player co-op as well. Please give it a spin and try it out.

PC Engine Fan X! ^_~

[Light1000]

Touhou 8
Crimzon Clover World Explosion

[D]

Sonic Wings Special
26 ships unlockable
branching paths, many levels

[Ixmucane2]

Xeno Fighters R has both many long levels (played individually or linked in a campaign with some branching) and many substantially different player ships.
I tend to practice the same level with different ships, attempting to find a suitable one.

[Necronom]

R-Type Final 2 has around 100 playable ships, including dlc.
Many well designed arcade stgs/shmups make "multiple playthroughs fun" by design.
My favourite (with lots of 'stuff'): Raiden Fighters Aces

[Steven]

Literally everything that has score (I know there is at least one that doesn't but I forgot its name) if you play for score. Well, except things that have infinite score design flaws.

[AGermanArtist]

Would Qute's output fit?

[NeoStrayCat]

Would Qute's output fit?

Probably, yeah, however...

Eschatos requires you to play a lot to with an experience system to unlock content. Judgment Silversword uses your score to unlock content. Ginga Force and Natsuki Chronicle has a shop system to unlock everything on there, which will take a while.

[davyK]

RType Final has a pile of content to unlock.

Someone has already mentioned R-Type Final 2 but I don't know how much that's based on the first one (not to mention how ridiculous a concept that is - Final 2!!!)

The Mushihimesama port has a pile of content in the form of modes etc albeit unlocked from the start, same with DoDonPachi DFK (aka Resurrection). DFK's content is bewildering there's so much of it.

The Raiden Fighters games have a pile of replay because of the ship selection. The later Parodius games are the same - especially Oshaberi Parodius which also has special score attack modes.

[beer gas canister]

Mecha Ritz, The Void Rains Upon Her Heart, Angel at Dusk

[Daytime Waitress]

I tend to gravitate towards games where ships/characters are so radically different that it feels like you're playing a whole other game depending on your selection, so I'd recommend Sophstar, Alfa System's Castle of Skikigami 1,2 & 3 (and also "4", Sister's Royale) and Cotton Rock'n Roll. The latter also leaves stage selection entirely up to the player, so you can stitch together a run entirely suited to your playstyle/prefence.