For Guilty Spark, it's more than a rumor; he actually survived the events of Halo 3.
For Didact, it's more than a hint now, because a 343i rep said that we had not seen the last of the Didact after Halo Escalation.
But I'm not a huge fan of people getting brought back to life. If their defeats were originally ambiguous (like Didact in Halo 4), then sure, bring them back, because it wasn't for sure they were ever dead.
With Guilty Spark, I'm surprised they made the claim he survived, because in Halo 3 you shoot him with a Splaser until he literally explodes, so I don't know how there were any remains of his chassis. Cortana is another example of someone they shouldn't have brought back. Maybe because these beings were artificial, 343i thought there was some inherent ambiguity in whether they could ever actually "die".
If they bring back someone who is obviously dead, like Johnson or Noble Six, I might just be done with Halo.