I wouldn't go expecting anything difficult with Pumpkin/Frost Moon equipment and/or Fishron Gear.
Not too many players can handle Frost Moon solo, and I doubt they're going to tune the Martian Event to where it'd be difficult with the best stuff 1.2.4 has to offer, because then players would face a huge ridiculous grind to even have a chance of starting the 1.3 content.
Now, they could nerf Fishron/Moons a little to make them more accessible to solo players (kinda like they did 1.1's hardmode gear in 1.2) and they could add new stuff one can get Post-Plantera, and/or make some of that stuff a smidge easier to get maybe.
I don't see them putting a huge brick wall in front of players before they can experience the new content. They didn't in 1.2; Plantera is not THAT large of a step up from the Mechs, for example. And they gave you Chlorophyte gear and Turtle Armor before Plantera, so you could prepare for him if you happened to need better stuff.