The lack of scaling gives them pretty poor utility for Sentinels relative to Scouts, and is partially responsible for the current meta of bricked Scouts.

I'm sleepy so I'll cut to the chase; rather than a plate/ext offering a flat bonus, why not have them offer a bonus based on a percentage of your base HP?

That way armour suits tank armour better than shield suits, which tank shields better than armour suits do.

At the moment this mightn't be a desirable change owing to the absence of valuable high-slot mods for armour tankers ( limited to shield extenders for the time being), but once modules are added/moved, I think this'd be a good way of balancing the frames.

Obviously the proportions would have to be small enough that Sentinel HP doesn't skyrocket, but large enough that Assaults and Logis get a meaningful bonus. The small base totals of Scouts would make plates far less desirable and implicitly encourage more 'scoutly' play.

I don't know what that is.

Percentage is a bad idea. How do you balance it between all the racial suits? Minmatar who are naturally less tanky would be even less tanky because their bases are lower. It would also make modules almost useless for certain races. If Shield modules give % base it becomes extremely unhelpful for Gallente because of their lower shields. Same thing with Caldari and Plates.

If that was the road to take, it would be better to keep the flat amount but the suit has some kind of decimal modifier; say .7 for Lights 1.0 for Mediums, and 1.3 for Heavy suits. So a Complex Shield Extender would give 46.2, 66, or 85.8 depending on the suit's weight class.