One interesting concept that can be derived from that team is you can turn it into a Trick Room team, swapping out Vaporeon for Empoleon (with Rocks and Defog) then Donphan for Porygon 2 as a secondary TR setter.
Cofagrigus and Mega-Ampharos are both very potent TR sweepers and you can keep your Fire/Water/Grass core by using Vileplume in place of Roserade. Vileplume itself has a solid 110 spA and 50 speed is good under TR, it has a similar movepool to Roserade and slightly more tanky on the physical side if you're worried about Scrafty. Arcanine can be kept as a fast cleaner when everything is weakened especially with that Extremespeed it can easily pick off weakened targets, not to mention its strong coverage. As much utility as Knock Off has, it...doesnt do much damage when used by Cof, consider Nasty Plot as an alternative..........
Sorry if that sounded like you need a team change, but as your current team goes, it looks solid. Consider sleep powder over leech seed if you don't use it much, sleep is always spammable for the first poke and Roserade discourages grass type switch-ins. Mega Ampharos itself is meant to be a slow and tanky, I'll assume those speed EVs are there to let it outspeed base 60s (which includes the base 58 Scrafty if it does not invest in speed), the next speed tier you will face is a fellow Vaporeon or Umbreon at base 65 which you will then need 164 EVs...though you'll need full speed investment and a timid nature to reach the major threats in base 80s, which I think is not worth. Currently with those EVs you can outspeed up to neutral nature base 60s which is these guys: Empoleon, Scrafty, Swampert, Trevenant, Machamp, Porygon 2 (If I miss any, sorry). If none of these guys are a threat then feel free to invest everything into HP for bulk. If you really need it to outrun Aggron and Blissey then 84 EVs would be nice.
An interesting Roserade I used before was a Technician one, exactly the same EVs and items, just Magical Leaf, HP Fire (Both tech boosted) and Sleep Powder. Slude Bomb remains unchanged because it has nothing else better to use. Looks weird, but you'd be surprised how hard it can hit.
Sorry again for the long post, I'm just a teambuilding nerd, heres some ideas for you think about.