Arbitrary bonus BST: +50
Adding abilities: As long as it's not shit like Huge Power and makes some sense, go ahead
Making custom abilities: Sure, why not
Expanding Movepools: Don't go overboard
Hard Mode: Don't slap a form of recovery on every defensive pokemon.

Bronzong
Type: Steel/Psychic
Ability: Heatproof/Levitate//Heavy Metal
HP: 67 -> 102 (+35)
Atk:89 -> 104 (+15)
Def:116
SpA:79
SpD:116
Spe:33
Learns: Heal Bell
Bronzong suffers from "Metagross, but worse", So I just focused on making it bulkier than base metagross and gave it enough attack that it's not completely passive. And the fact that the 'Bronze Bell' pokemon doesn't learn heal bell is stupid