Part 1: http://www.mangago.zone/read-manga/akai_ito_no_shikkou_yuuyo/
Part 2: http://www.mangago.zone/read-manga/kanawanu_koi_no_musubikata/
A MUST READ MANGA!!!!
Dunno the genre you want, if its BL/Yaoi:
Literally red thread:
http://www.mangago.zone/read-manga/akai_ito_no_shikkou_yuuyo/
http://www.mangago.zone/read-manga/kanawanu_koi_no_musubikata/ - this one is the sequel
Soulmate/fatedpartner (omegaverse mangas):
http://www.mangago.zone/read-manga/kiraide_isasete/
http://www.mangago.zone/read-manga/kashikomarimashita_destiny/
http://www.mangago.zone/read-manga/there_s_no_way_this_is_fate/
http://www.mangago.zone/read-manga/2ban_me_no_alpha/
http://www.mangago.zone/read-manga/shounen_no_kyoukai/
are there any manga or manhwa with red thread or soulmate theme?