I’d argue that it decreases our chances. It all hinges on your theory that a Golden Sun is in development. But we don’t know that.
And even if a Golden Sun were in development I can assure you Isaac would not be an AT right now. He’d been in the base roster right now.
No, you're completely ignoring one major factor:
Time.
It takes a lot of work to include a character in Smash, and this game has clearly been rushed. Its not hard to see. Based on all factors we've seen, it seems very likely that characters got cut, which made them add more Echo Fighters and promoted Isabelle from Echo to full-fledged newcomer (despite only having like three unique moves). Especially with Isaac being such a massively complex character.
Chrom is another huge example of this. Robin's FS wasn't changed at all, despite many other characters getting changed. Chrom was added too late to change it.
If GS wasn't coming until next year, it gives all the more reason for him to be cut and postponed to DLC if they're running out of time for characters. That'd also explain Isaac's AT coming so late. He wasn't planned to be an AT, but they didn't have time to finish him before release, so they worked on the AT instead.
Obviously, this is only one possibility. But I feel its far stronger than you're suggesting.
No. It would increase the chances of that games protagonist. Which we can assume would be either Mathew or Felix’s child.
No it wouldn't because Isaac is the fan favorite character.
On the topic of Golden Sun's trademark getting renewed, I want to throw out an idea. Maybe Golden Sun's trademark was renewed when it was so Sakurai and his team could add Isaac as an assist trophy again. That would explain why Isaac was revealed at the last second to crush our dreams. I don't want to rain on anyone's parade, but I don't think we should look at the trademark renewal as a sign for Golden Sun 4.
Then you'd have to explain why Custom Robo was not renewed, despite having a Mii fighter costume in the game.