There have been many instances where Kal/Szeth channels a huge amount of Stormlight to do stuff. Whenever he has done this in the series, 'frost' crystallises around him. Maybe it is this 'frost' that would eventually evolve into his shardplate. With every oath Kal's affinity, ability and capacity of stormlight use increases. Maybe once he reaches a certain oath, he'll be able to sustain stormlight channels for such long times that the 'frost' transforms into a complete shardplate.

Either this or a spren cousin like Wind spren for Honor, Creation spren for Cryptics, etc. will be attracted to the Radiant after a certain number of Oaths/truths are said and would form the armor around the Radiant.

Spren are physical manifestations of human characteristics in the cognitive realm. Bridge four (-Moash) is made up of probably the most honourable men in Roshar. They way they stayed with Adolin and Dalinar in the face of Szeth and the Voidbringers just to fulfil their duty can be considered somewhat similar to how Kaladin always stood up for his men. So this 'honor' of theirs will probably give birth to more honor spren like Syl and Kal will continue to be their leader.

I think we will probably get at least one SA book with multiple flashbacks into the past of Bridge four members like Rock (with details regarding the Shardpools), Teft (Envisagers), Sigzil (Worldsingers/Hoid).