imo, With acceleration in the game, Its not very necessary to give collosal dragon a buff.
Well Armagio is still over it in terms of Rage Potioning and Accelerating/Overdriving
when upgraded it isnt that much. they both have 30 hp when upgrading , in this case collosal dragon is over it in terms of power, it would be even more if it wasnt for the cost.
but that in itself doesnt need a buff.
Actually, I think Colossal Dragon is less UP than Massive Dragon... They both are balanced in the atk/hp/cost thingy, but Massive Dragon's niche is the same as Armagio's, thus that +15 hp makes it unneccessarily more expensive. Colossal Dragon is awesome with Acceleration/Overdrive, and a Colossal Dragon/Acceleration deck does better against RT than an Armagio/Acceleration one, because the Dragons doesn't need the Acceleration to deal damage.
Whatever, Massive Dragon could have 20 hp and get his cost reduced by 1
, as to have a niche on his own (semi-high hp and semi-high atk).
Also, any card that buffs hp would need to take in account atk too to buff this dragon. Otherwise it would end the same as ever, Armagio overshadows it... Acceleration is a clear example of a buff to this Dragon, and I think it just need a minor buff, and maybe what I said of making it 8|20 and costing 10