I think you should focus on back-stories first, then from there, develop the technology and characters relationship.
It is more natural this way rather than just jumping all over the place.
For example:
Technology:
Uchigane > Tenshi (Uchigane + Raphael design for more speed, an excuse why Byakushiki look drastically different from other Japanese Unit) > Byakushiki (original)
(Due to (original) Byakushiki being scrapped):
Uchigane> Uchigane Juusoubi (Heavy Armament) > Uchigane Nigata (half-completed)
Relationship:
OC admire Tabane > OC meet Tabane > Tabane trolled> OC dislike Tabane > OC deliberately keep sabotaging Tabane's plan > Now the feeling is mutual (read: "THIS IS WAR!!".)
Rather than "destined for each other" or "chosen pairing", go with your imagination, put character A with character B and see if there is any chemistry between the two.
If there is any similarity in interest, they would probably be attracted to each other.
For the Canon character, probably give the reader one a good reason/excuse why the harem like Ichika.
Not sure if it is a sound advice because I don't use it.... which is why I am going nowhere.
Still trying to get a feel of Infinite Stratos characters.... Just feels kind of hard to get a grasp the girls' personality.....
Mostly because they seldom interact with other characters to show us how they are like beyond swooning over Ichika.
Except for.....
Maya and Chifuyu (Even though they are talking about Ichika but it shows Maya's personality beyond being a klutz and Chifuyu being a little over-protective.)
Charlotte and Laura (a side-story about them together)
Probably Kanzashi too....
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Edit: It is easier to see a character's personality when they are not focusing on their love interest...
Take for example:
Super-Lesbian Kuroko(To-aru): If you only see her from Mikoto's eyes, you only see a crazy lesbian bitch but if you see from her and characters's (other than Mikoto) POV, you could see that the girl actually is more than just a Psycho-Lesbian.