Believe it or not, X-2 had a lot of scrapped ideas as well, though not on the same scale as FFX. Because the world of Spira was already presented to us in X-2's predecessor, that was really all the material they could work from. So unfortunately, there's no giant hoop-earrings Auron or Ronsos wearing futuristic clothing here.
However, there were a lot of unused concepts for dresspheres. And I mean, a LOT. And it's all in Japanese, so I have to translate it all! Yay! :D
Fun facts: In the original (2001 version) Japanese version of the 15-minute Eternal Calm prologue, the sphere which Rikku shows Yuna is almost completely different. Instead of a slightly broken picture showing Shuyin trapped in a dark cage, it instead shows a really gritty and low-quality picture of Tidus trapped in a cage, which oddly, looks like the one which Jecht was held captive in when he first came to Spira. This could imply that, at first, the writers had the idea that Tidus was indeed still in Spira somehow, or maybe it was because they didn't make the Shuyin model yet.
As of the audio drama, Final Fantasy X-2 had quite a bit of names before they settled on the '-2' extension. One of these names was Final Fantasy X+, which is clever because + looks similar to the Japanese symbol, '十', which means Ten. Another name was Final Fantasy X ReLive!, which could come from the fact that the game would be centred on reliving Spira after Sin's defeat. At around 17:07 in the audio drama, there's another name with three symbols in boxes, but I can't make it out. Weirdly enough, the Amano artwork is all the same.
The original draft for Shuyin's role in X-2 is that his existence was preventing Tidus' return to Spira. His role was changed since the writers thought it would make it look like Yuna was sending Shuyin for selfish reasons. This could explain why Tidus' and Shuyin's connection isn't outright explained, since it was scrapped along with the original idea. This could also explain why Tidus' resurrection and method of such seem sudden and out of place compared to the rest of the game.
Keep in mind that my Japanese is very scarce, so at best, I will only be able to recognise katakana, some hiragana, and very little kanji. I'm bound to make at least a few mistranslations, so if people could help me out, I'd be truly grateful.
Airship Battle
(http://img1.wikia.nocookie.net/__cb20100207035056/finalfantasy/images/8/87/X2pre-concept_3.jpg)
Early concept art for X-2. Depicts Yuna, Rikku, and an early version of Paine fighting what is guessed to be an early version of Major Numerus. The location of the fight is unknown, though because it's in the sky, the buildings might be an early version of the Fayth Ruins at the start of X-2.
Battle Concept
(http://img4.wikia.nocookie.net/__cb20100207035056/finalfantasy/images/0/0e/X2battleconcept.jpg)
Diagram of X-2's battle system. This might imply that the battle system was similar to X's as it had turn-based combat and the Overkill combos. Also depicts Yuna having shorter hair, Rikku having her hairstyle and clothes from X and another early version of Paine.
YuRiPa
(http://img4.wikia.nocookie.net/__cb20100207033020/finalfantasy/images/5/5e/Concept_FFX-2_YRP.jpg)
Concept designs for YRP's 'intro' seen in numerous points in the game.
Lenne Concept
(http://img1.wikia.nocookie.net/__cb20100416205427/finalfantasy/images/0/05/Lenne-concept.jpg)
Early designs of Lenne. Clothing appears to be more detailed and modern than in the final game.
Farplane Design
(http://static2.wikia.nocookie.net/__cb20080314050857/finalfantasy/images/3/3f/FFX-2_Farplane_concept.jpg)
Drawing of Yuna at the entrance of the road to the Farplane.
Rikku Design
(http://img3.wikia.nocookie.net/__cb20100207033022/finalfantasy/images/7/7f/Concept_FFX-2_Job_Rikku_Thief.jpg)
Early design of Rikku's Thief dressphere, meaning that this could have been her original X-2 design.
Leblanc Concept
(http://img3.wikia.nocookie.net/__cb20100416205305/finalfantasy/images/7/7a/Leblanc-concept.jpg)
Early design of Leblanc. Has slightly different clothes, spikier hair and appears to be rather chubby. She has a whip instead of a paper fan.
Festivalist Concepts
(http://img3.wikia.nocookie.net/__cb20110818152645/finalfantasy/images/4/42/Festival_goer_concept.png)
(http://img1.wikia.nocookie.net/__cb20110818152957/finalfantasy/images/2/22/Festival_goer_concept2.png)
(http://img3.wikia.nocookie.net/__cb20110818153147/finalfantasy/images/5/53/Festival_goer_concept3.png)
Before International + Last Mission was released, Square Enix held a competition to design a dressphere concept that would make it into the updated game. A small child sent in pictures of YRP dressed up in traditional Matsuri wear, and this promptly became the Festivalist dressphere.
Dark Bahamut
(http://img1.wikia.nocookie.net/__cb20090708052235/finalfantasy/images/5/5d/FFX2_bahamut_artwork.png)
Concept art for Dark Bahamut.
Dressphere Designs
As previously mentioned above, there were a LOT of unused dressphere designs, all by different artists and such. Some have their own individual scans, others are viewed in a sort of 'sketch dump'. To make things easier, I will be firstly displaying the officially named dresspheres, and posting the 'sketch dumps' at the end.
If possible, I will also provide rough translations on the abilities and descriptions of said dresspheres. Also, since I will most likely be making lots of mistakes and corrections, I will just provide the names below the picture as opposed to big bold letters on top of it.
(http://img2.wikia.nocookie.net/__cb20110818161352/finalfantasy/images/b/b3/Yuna_concept4.png)
Name: 舞 - Dance (Describes a traditional Japanese dance, consisting of slow, quiet movements. Could be an early concept of the Songstress.)
(http://img1.wikia.nocookie.net/__cb20110818161056/finalfantasy/images/e/eb/Matador_dressphere_concept.png)
Name: Matador
(http://img2.wikia.nocookie.net/__cb20110818161532/finalfantasy/images/2/29/Dressphere_concept2.png)
Name: Hurūtu Pārā - Fruit Power
(http://img1.wikia.nocookie.net/__cb20110818160833/finalfantasy/images/8/84/Yuna_concept3.png)
Name: Douriima - Dreamer
Ability:
-Seems to be mathematics on how the HP and MP works.-
(http://img4.wikia.nocookie.net/__cb20110818161754/finalfantasy/images/a/ac/Dressphere_concept3.png)
Name: Pikuto Mansā - Picto Master?
(http://img1.wikia.nocookie.net/__cb20110818155135/finalfantasy/images/9/9a/Dressphere_concept.png)
Name: Horā Doresu - Horror Dress
(http://img1.wikia.nocookie.net/__cb20110818154313/finalfantasy/images/5/5f/Concept_art_from_ultimania2.png)
Name: Burututi Gāru - Birdie Girl
Writing on the top right: Direct prize
[Birdie Girl]
--------------
-Kanji remains to be translated-
[Prize goods:] FFX-2 Paine's necklace
(Based on layout and translations, this could have been a winner for a dresssphere-designing contest.)
(http://img3.wikia.nocookie.net/__cb20090507225651/finalfantasy/images/7/7d/YunaGunblade.jpg)
Name: Bureedo Gannā - Blade Gunner (Based on the Gunblade from FF VIII).
(http://img2.wikia.nocookie.net/__cb20110818161956/finalfantasy/images/f/fa/Geomancer_concept.png)
Name: Geomancer
(I'm kind of disappointed that this wasn't used. This is by far my favourite dressphere, concept and visual-wise.)
Will update in spare time. Feel free to help out with the translations!