have you ever tried scarf heatran? it's not used as often so it's often unpredictable but it's been working well for me, been using it since 5th gen lol. i don't like specs tran. if you wanna go for a fire specs mon choose chandelure or something. raw STAB damage that hits hard.
heatran as a lead gets rekt by someone like landorus and a few other leads which is super common unless you have air balloon which is a waste imo. i mean you can have it as a rock setter but it'll have to be brought in later.. at least that's just my experience with it.
here's my set for it:
Heatran (M) @ Choice Scarf
Ability: Flash Fire
Shiny: Yes
EVs: 24 HP / 8 Def / 216 SpA / 8 SpD / 252 Spe
Modest Nature
- Lava Plume
- Flash Cannon
- Ancient Power
- Earth Power
this is just my opinion but sylveon is an amazing cleric and i think that's pretty much her only niche in ou anyway, using her as a specs user there are honestly so many better options. if you really want a fairy that can hit hard and sweep go with clefable or something. life orb+magic guard is op. 1-2 calm minds & flamethrower/moonblast with full spA invested is amazing.
also why not just go for landorus scarf lead w/ rocks? why is mamo needed here, like what is its purpose in this team?
i don't think it would hurt to try mega scizor in this team. i feel like latias is just fine without a mega and in fact, could be used as a defogger as she's one of the best defoggers in the meta right now along with latios. with that you could also replace starmie since rapid spin won't be needed anymore. or if you wanna go for a good mega, try mega altaria and scratch the idea of clefable for a sweep.
there's literally so much you can do for this team, and seeing you're going for like mostly offensive it's really up to you what route you wanna go with this. just throwing some ideas out there though, i hope it helps some.
also you should edit the op with your changes.
Why Lava Plume on Scarf Heatran? Sure, you can fish for burns, but the raw power of Overheat or Fire Blast is preferred simply because Choice Scarf Pokemon are weaker compared to Life Orb variants. Despite being weakened by a Modest/Timid nature, Stone Edge is preferred over Ancient Power in order to hit specially defensive Charizard Y and Talonflame, as well as Volcarona harder:
0- Atk Heatran Stone Edge vs. 248 HP / 0 Def Talonflame: 316-372 (88 - 103.6%) -- 25% chance to OHKO
252+ SpA Heatran Ancient Power vs. 248 HP / 252+ SpD Talonflame: 264-312 (73.5 - 86.9%) -- guaranteed 2HKO after Leftovers recovery
0- Atk Heatran Stone Edge vs. 248 HP / 0 Def Mega Charizard Y: 292-344 (81.3 - 95.8%) -- guaranteed 2HKO
252+ SpA Heatran Ancient Power vs. 248 HP / 132+ SpD Mega Charizard Y: 208-248 (57.9 - 69%) -- guaranteed 2HKO
0- Atk Heatran Stone Edge vs. 0 HP / 0 Def Volcarona: 340-400 (109.3 - 128.6%) -- guaranteed OHKO
252+ SpA Heatran Ancient Power vs. 0 HP / 4 SpD Volcarona: 276-328 (88.7 - 105.4%) -- 31.3% chance to OHKO
Ancient Power still has merit, though, such as still hitting Charizard X if you choose to run Hidden Power Ice (which is another option to hit Gliscor and Therian Landorus harder than Fire Blast/Overheat).
Tl;dr:
-Fire Blast/Overheat
-Flash Cannon
-Earth PowerHidden Power (Ice)
-Stone Edge/Ancient Power
Nature: Timid
EVs: 28 HP/4 Def/252 SAtk/228 Spe
Item: Choice Scarf
With a Timid nature and this EV spread, Scarf Heatran outspeeds Mega Lopunny, something your current spread fails to do. Seriously, what does your EV spread accomplish?