>>281368691read
>>281368678 for more info, but I might as well give you the full picture
The first "draft" of Fate was what later morphed into the Fate/Prototype subseries. Nasu however calls his draft Old Fate, as Prototype still diverges from it somewhat. An otome VN with the female magus Ayaka, her servant Arthur, her crazy sister Manaka, and a few other assorted characters, some of which would end up being converted into FSN characters. Old Fate's story only managed to reach the clash with Assassin, who was still Sasaki and managed to get ported over to FSN's framework with little to no changes, similarly to Cu and (to an extent) Gil. In the end, an LN with a female protag would've simply not been a hit, and Takeuchi knew that.
Afterwards, the story more of less became the FSN you're acquainted with. With the exception that the route structure would've been more like Tsukihime's, with more routes that are shorter and a bit more half-baked. Illya, Rider, Caster were the big contenders, although most of their stuff ended up being molded into FSN and FHA's structure in one way or another. The parts where Rider/Caster would've become Shirou's servant, however, were completely left out of the picture.
Of course, the Shinji route also has to be mentioned, seeing as despite it being another serious consideration, 99% of it never managed to materialize. Shirou would've teamed up with Shinji, getting to become uneasy but understanding allies and at some point going head-to-head with Saber and Rin. Kirei and Zouken would've also played a major role as rivalling antagonists. In the end, the only remnants of Shinji's deeper (yet still broken) personality would surface in FHA and the UBW anime's drama CDs.
The rest goes as told, more or less. People meme that Prillya's the Illya route, and they're not exactly wrong when considering Oath Under Snow's existence.