If you want to keep some kind of penalty on spellcasting from armor, the easiest way would be to use armor check penalty. Simply make it apply to magic attack too. Not being proficient with armor doubles the armor check penalty. Much easier than cooking up a new system.
Two sorcerers in my campaign tend to wear armor without any penalty on their spellcasting and I haven't found it to be overpowering. They've still been biting the dust or close to death many times.
Two sorcerers in my campaign tend to wear armor without any penalty on their spellcasting and I haven't found it to be overpowering. They've still been biting the dust or close to death many times.