I like Mega Aggron. The only problem is that it struggles with entry...Steel Rock isn't a great starting type and if you misread an attack it can be forced into a SleepTalk cycle pretty quick.
Cress as you mentioned is also one of my favourites. It works well with Porygon2 and Sylveon I've found. Always kind of secretly hoped it gets Heal Bell or something like it sometime just so it becomes a hilariously fat cleric.
What does this topic think of defensive Arcanine? I am tempted to try out a non-standard FWG core and Arcanine seems to work well as an alternative to Heatran. Intimidate is always useful and it has access to pretty much the entire core defensive Heatran set aside from Stealth Rock (Roar/Will-O-Wisp, and bulky enough stats). With Morning Sun it can regenerate fairly well...it'll struggle against TTar but I guess so did Heatran. Killer is that it regains that rock weakness in comparison and continual cycling it in for 25% damage will wear it down...but it looks like a nice Pokemon to try out.
Defensive Fire types in general have been a recent focus of mine, purely and solely because of their application against Mega-Mawile. They obviously won't appreciate the possibility of an incoming Knock Off, but resisting the Play Rough is regardless a huge asset. Arcanine is one such example and it really didn't underperform when I tried it out.
-1 252+ Atk Huge Power MegaMawile Knock Off vs. 252 HP / 0 Def Arcanine: (42.18 - 49.73%)
-1 252+ Atk Huge Power MegaMawile Play Rough vs. 252 HP / 0 Def Arcanine: (28.9 - 34.37%)
Both give you a free chance to fire off a will-o-wisp or use Morning Sun if you don't expect them to sub up.
However, going full bulky (and a little speed creep) will make this pokemon pretty weak.
0 Atk Arcanine Close Combat vs. 248 HP / 48+ Def Heatran: (40.51 - 47.79%) -- Chancy 2hko after SR and Leftovers
Other Options for bulky or bulky-ish Fire types that can possibly switch into MegaMawile might include Charizard-Y sans hazards (doesn't take Knock Off damage), Heatran, Chandelure, Moltres, Rotom-H, Entei, Victini, Talonflame, and ... Magcargo? Charizard-X is bulky also.
Bulk is a relative word: if Rotom-W can be bulky with a base 50 HP stat, then it's not impossible for a fire type to be bulky as well with the right investments.
Before abilities, Entei has the largest physical bulk if I'm not mistaken. After intimidate, Arcanine is physically bulkiest.
252+ Atk Huge Power MegaMawile Play Rough vs. 248 HP / 0 Def Entei: (36.72 - 43.18%) -- 53.91% chance to 2HKO after Stealth Rock
252+ Atk Huge Power MegaMawile Knock Off vs. 248 HP / 0 Def Entei: (53.34 - 62.81%)
Mawile is just fckin strong, like too strong, and all of those above take serious damage coming in, especially from Knock Off.
Forcing a bulky fire into my team, regardless, is an ongoing mini-project. At the moment Rotom-H is my favorite.