This team already has quite good natural bulk, but struggles against fire attacks with M-Scizor and Magnezone both KO'd by fire moves. It also lacks a Z-Move user, and doesn't have an easy time against stall (Zygarde helps, but can't break unaware users alone). Enter Heatran:
Heatran @ Firium Z
Ability: Flash Fire
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Magma Storm
- Earth Power
- Taunt
- Toxic
Against stall, with Firium Z, Heatran can break M-Sableye, a mon this team slightly struggles with, and toxic can cripple Gastrodon, Unaware Clef or Quagsire, makes Chansey easier to take out, and is a general nuisance. One could replace Toxic with Solar beam to give Tran a nuke against the aforementioned bulky water types, but I though Firium-Z was more consistently devastating a move.
Outside of facing stall, few mons can switch into Heatran. With Firium-Z it can nuke just about anything for good damage, and the trapping and passive damage off Magma Storm provides solid damage to just about anything that comes in. Heatran also appreciates the presence of Mega-Scizor and Tornadus-T as reliable switch-ins to ground type moves. In addition, the only reliable switch in on this team to Heatran itself is Zygarde (other than the rare HP Ice variants that have popped up recently), and adding max speed Heatran improves the teams matchup with opposing Heatrans, adding a second mon that is at worst a coin flip vs opposing Heatrans, and a reliable switch in to fire type moves.
Finally, the team lacks a taunt user, and can struggle a bit against bulky calm mind sweepers (Latias especially, but also Reuniculus and Clefable to a lesser extent). With taunt, toxic, and trapping Heatran can do good work against a variety of these mons too.
All in all, Heatran can fill in some of the gaps on this team, and would be a solid addition to it.