Free hero for backers and ultimate pack is definitely more rewarding than what was originally offered (two campaign chapters and a fog of war skin). As to speculating what that hero might be according to the roadmap we will be getting two coop heroes later in 2024.
I speculate one of those heroes will be Ryker on account that his model is already functional in the campaign. The second hero I believe will be the arch angel with a sword that we see on the new promotion art. It makes sense that celestials will get another hero as they only have Auralanna for coop compared to the other two factions.
I personally hope the free hero will be the celestial hero as it makes up for the fallen celestial that Warz should have been in the first place.
No spoilers, but I hope you'll be pleased with our next hero...