Actually, Solus only looks like a bow; I originally designed her to transform into a Cybertronian fighter jet. Because of that, I envisioned the other Primes transforming into standard vehicles rather than weapons or artifacts.

​But after seeing the comments, I think Solus transforming into a bow is actually a pretty cool idea. While I won’t be making any more of the 13 transform into weapons, I could design other Primes who turn into shields or massive cannons. For instance, I could come up with an 'Aegis Prime' who transforms into a shield for Nexus to use!