I think mienshao is the best choice so far but I honestly don't think we have found the perfect fit yet.
You are now my new favorite person.
Heatran makes this team even more weak to Starmie and Thundurus-T, as well as the not-so-seen Abomasnow.Going to defend Heatran, as I think it is the most reliable contender for the third spot.
First of all, I agree that air balloon may sound stupid if we already have 2 ground immunities. However, AB isn't only useful for switching into a predicted ground attack. AB can also be very usefull to wall certain pokemon such as SubToxic Gliscor (who really threatens our team, stalling out Rotom-w's Hydro Pump) and Hippowdon. Other useful uses are beating Garchomp 1 vs 1, forcing Landorus to rely on Focus Miss to defeat it (only do this if anything else fails, of course), countering Dragonite/Salamence carrying Earthquake and of course its spike immunity. I can however see your point about AB being redundant. Though this is where leftovers/life orb come in. With leftovers, Heatran has a much easier time staying alive. The calcs below also show that. Life orb gives you a lot of power, as LO Earth power 2HKO's the most specially defensive Tyranitar. LO HP ice deals 67.44 - 79.4% to standard Latios and 48.62 - 57.14% to Latias, which means Heatran 2HKO's one of the best special sponges of the tier.
And to the person who said Heatran can't take outrages well (these are assuming lefties are held):
252 Atk Salamence Outrage vs. 0 HP / 0 Def Heatran: 96-113 (29.72 - 34.98%) -- guaranteed 4HKO
252+ SpA Heatran Hidden Power Ice vs. 0 HP / 0- SpD Salamence: 452-532 (136.55 - 160.72%) -- guaranteed OHKO
252+ Atk Dragonite Outrage vs. 0 HP / 0 Def Heatran: 105-123 (32.5 - 38.08%) -- 1.46% chance to 3HKO
252+ SpA Heatran Hidden Power Ice vs. 0 HP / 4 SpD Multiscale Dragonite: 168-198 (52.01 - 61.3%) -- guaranteed 2HKO, guaranteed OHKO if Multiscale is inactive.
+1 252+ Atk Dragonite Outrage vs. 0 HP / 0 Def Heatran: 156-185 (48.29 - 57.27%) -- 46.48% chance to 2HKO
See? So basically, even if Heatran switches into Dragonite on the DD AND Multiscale is still intact (which you shouldn't let it do anyway), Heatran still wins more than 50% of the time. Something Ferro or Forre can't say, as Dragonite often carries Fire Punch (+1 252+ Atk Dragonite Fire Punch vs. 252 HP / 88+ Def Ferrothorn: 372-440 (105.68 - 125%) -- guaranteed OHKO).
In the end, I prefer the leftovers as it is your only form of recovery, and Heatran still has plenty of power. Heatran IMO is a more reliable check to dragons than Ferro, as Fire punch is more common on Dragonite than Earthquake. (moveset statistics from smogon: Fire Punch 49.930% and Earthquake 36.677%)
So, bring on your counterarguments