If they were to bring in Balder, I wouldn't want them to acknowledge him as a son of Odin (that was a pretty recent addition to the character, relatively speaking). I don't know that he's necessary at this point, but in a mostly-not-on-Earth Thor 3, I think there could be a place for him. In my Thor 3 treatment (link in my sig), I would explain his absence from the other films as a result of him being an ambassador to Nornheim, and therefore off world until now.
^ Great ideas. That's the real point, isn't it? There are a ton of creative ways that anything can be done in these films. Nothing is off limits as long as it is done correctly. Saying it's too late for Balder is like saying it's too late for Surtur, or Amora, or anyone else to show up.
I very much would like to see Balder. It's not too late for any of these characters. Beta Ray Bill didn't come into Thor lore for 20 years.
A simple reason Balder has never been mentioned is because Thor doesn't even know about him. Odin hid him to prevent Ragnarok. Odin keeps secrets ALL THE TIME. That's what he does. Maybe he even made a deal with Karnilla who's obsessed with Balder or worse Hela - perhaps to induce a false Ragnarok to prevent the real one. Maybe Thor goes to Hel for Odin and wins back Balder too. I dunno. I don't write these films but I'm just spitballing here. Possibilities people!