The first stage would be to have a gun (rail or otherwise) that can fire further than the range of carrier based airpower or missiles and have it very cheap in comparison.
There wouldn't be any sense of triple turrets or a particular calibre, but rather what would be needed for a land attack mission of ~1200km. Said guns would be deployed in twin mounts at maximum, to fit with modern gun emplacement thinking/strategies (fewer launchers, more rounds and capacity). Therefore you could be looking at a ship with one large twin mount fore and one large twin mount aft. The gun, if not a railgun system, could be based on the US Army's Strategic Long Range Cannon, should that ever see the light of day; whilst the calibre is not currently known, we'll call it 12" for now.
Displacement is dependent on a number of factors, but it would be at least 15,000t and at most 25,000t, based on those general parameters. To that basic hull, we can then chuck on CIWS, self defence missiles, radars, possibly tactical lasers and what not. Whilst there would be no question of an old style armoured belt, there would be passive protection on the level postulated for the CGN-42 in the 70s.
Once we've created this bastard great-great grandchild of the predreadnought, then there might, might be the possibility of applying that gun technology to the surface to surface/anti-ship mission. I can't see the rebirth of armour, though, as active protection measures seem to be both cheaper and more effective against current and projected threats.