I didn't see the Injustice aspect of the Hellboy rumor - I wonder if that's just speculation? It seems to be aligned with the other creator-owned figures like Savage Dragon and Shadowhawk, so it was my assumption he'd be a regular Hellboy.
Yeah, Spawn has never really had the cultural penetration of other superheroes - even indies like Hellboy or Invincible. I think it's partly because his origin is weird, gory, based in overly-complex religious mythology, and very '90s. But also, to some degree, I've always felt like Spawn, as a franchise, primarily existed as an excuse for Todd to make other things he thought were cool - toys, posters, cartoons, movies. Spawn has never really felt like a living, breathing character to me, for whatever reason.