I think he’s going to announce his wife is pregnant.
Any other guesses?
I don’t think announcing a pregnancy is the kind of thing that one would forecast or set the stage for, and I can’t see any sense in this announcement not being directly related to his campaign.
I don’t have a personal guess, but the media expect him to name a running mate (VP nominee), and that makes sense to me.
Hard to imagine that he has a VP all vetted and ready to go in April but I guess now’s the time for that Hail Mary pass.
Maybe he’ll announce that he’s going to be Trump’s VP 
His VP pick.
Has a candidate for the nomination, especially one trailing like him, ever announced a running mate this early?
Carly Fiorina has been spotted in Indianapolis.
He lowered his cholesterol with Crestor? One of his children isn’t his? He’s leaving his wife for a man?
Almost certainly his veep pick which will almost certainly be Fiorina which will almost certainly be quickly forgotten.
He self-identifies as female.
Rats! Beaten by mere minutes.
Instead, I’ll offer this one - he’s becoming a Buddhist.
Maybe he’s suspending his campaign after disappointing third place finishes yesterday.
No way. Not her. Can’t believe it would be her.
He can’t believe it’s not butter?
He improved his credit score using this one simple trick!
This would be the most yawn-inducing big announcement in the history of the universe.
He’s switching parties to become a Democrat :).
Reagan named his pick (Pennsylvania senator Richard Schweiker) before the Republican candidate was chosen in 1976. He ended up losing the nomination to Ford.
Yeah, he made that announcement in late July just prior to the convention.
I like it. That should actually BE his announcement: “Members of the press, Carly Fiorina has been spotted in Indianapolis. Good night.”
That would be hysterical!
He wants to give a preview of a dance he’s been working on.
It would seem the height of arrogance to announce your veep for a presidency that best evidence indicates is never going to happen. So that’s probably it.
Especially if it’s a practical joke by Trump, who then reneged on the offer or denied every making it.