So after a long thinking, I really come to believe that Shuichi's Sacred Artifact's Original Owner/motif seems to to be Gilgamesh.

The people that created Shuichi and his siblings based the Plan-A/B/C on the Geniuses of the modern time (which seems to not be from any other 23 worlds), but Shuichi and Duo were instead connected with Sacred Artifacts that originated from other Worlds, which we know that Duo is connected to Atlas of Olympus. If the perpetrator behind the game really wants someone from the World of Babel (Original name Enuma Elish), then Gilgamesh would be the logical choice for Shuichi to be connected to as he most famous figure of the Babylonian and Mesopotamian mythology given his Tablet Artifact's origin. On another note, Marduk being summoned to Tokyo could be the main reason why Shuichi being a failed Pillar since his situation is very similar to Jacob and Michael, the former can access it but the latter cannot.

Tokyo characters also generally have names with possible meaning that's associated with their Sacred Artifact's power/origin, such as Shinya ("Arrow of Truth" 真矢), Ikutoshi Oki (Oki sounds similar to the word "big" 大きい), and Kyoma Mononobe (There is a radical of kanji for King in his first name 匡, combining it with 真 and flipping it around would be "King of Truth" ). So for Shuichi's case it's Girugameshuu ギルガメッシュウ. Also, since it's stated that Duo is a nickname, there is a good chance that Shuichi named him after Enkidu.

Lastly, some of the characters have design that is a homage to other series (Looking at you, Mononobe/Romani/Solomon), so I wouldn't be surprised that his Tablet Artifact is a homage to Caster Gilgamesh's Bond CE.