Um do you have any thoughts about aforementioned possibly Not Straight Shiro?
I think it’s hard to say for sure since I don’t think we’ve seen either obvious Shiro expressing definitely-not-platonic interest in someone or scenes that can easily be read as Checking Someone Out.
(A point of some consternation for me: when people being emotionally close to each other is interpreted as “they’re too friendly to be platonic, they must be dating”)
We do know that Shiro is pretty comfortable with physical intimacy as an expression of affection, and, outside of the fairly recent in his life shakeup of his trauma, Shiro is a pretty confident, sure of himself person who’s comfortable in his own skin. These traits are retained to a degree even in the face of his trauma. That said, he’s also so responsible and duty-bound that I think in the past it’s unlikely he’d really focus on if he ran into someone who happened to be his type- so that might be something we see more as Shiro gets out of his own head a little and allows himself to relax.
I think because of that, if Shiro experiences romantic attraction, I think he’s fairly likely to be someone who might have had past relationships pre-Kerberos even if he might be single at present.
My personal headcanon of Shiro is him being bi, but I think that’s just a headcanon- again, we haven’t seen what I would call damning canon evidence of his sexuality at all. Prior versions of his character had a love interest in Princess Romelle, but even if Romelle exists in VLD we don’t know if she and Shiro will become an item, or if Shiro is even attracted to women.
Personally I’m pretty adamant against the idea that Keith and Shiro had a romantic relationship pre-Kerberos, mostly because of how much it seems that pre-Kerberos Keith was to a degree emotionally dependent on Shiro. Shiro was his only friend, the “only person who never gave up on” Keith, and we see that Keith, especially early on, is willing to go to any extreme to keep that relationship.
This is not a foundation I think a healthy romantic or sexual relationship- one where both people have to be completely at liberty to discuss and enforce their own boundaries- could be built on. Mostly, I do not think Shiro, with his strong sense of responsibility, would be comfortable starting a relationship with someone who so clearly put him on a pedestal like Keith did at first.