If s-mods became hullmod+ (I like the idea in concept), then ships would need a base OP increase IMO. Right now s-mods are basically ordinance expertise from last patch, and without that, ships will be very tight on OP.
Honestly, I think I prefer that solution. Having ships be balanced around their base OP rather than trying to balance around an unknown number of extra OP from s-mods seems much easier, and the hullmod+ concept seems like it could encourage a lot more s-mod diversity and cool builds.
I'll point out there's a significant difference between a base ship OP change, and the current player centric or explicitly harder than normal fight s-mod mechanic, in that you are making the player's fleet relatively weaker against typical enemies. If base OP is shifted to match what we can do right now with s-mods, well, that's like turning all NPC ships into 2 s-mod player ships.
While you could make up for it with a hullmod+ concept, the hullmod+ would essentially need to buff every single hullmod by a sufficient amount to be worth something like current s-mod savings. A frigate hullmod+ would need to be worth the base 5 or 6 OP you're paying, plus another 5 or 6 OP, to maintain current difficulty levels.
To make it concrete, let's take a Tempest. You might take hardened shields for 6 OP and hardened Subsystems for 5 OP. 50 base OP becomes effectively 61. Now imagine we tweak all Tempests to have 60 OP base line and s-mods no longer reduce OP cost. NPC's Tempests just became as strong as our old 2 s-mod Tempests. Now you need hardened shields++ and hardened subsystems++ to cost 6 and 5 OP, and be combined worth another 23 OP or so.
60+23-11= 72. 72/60=1.2, or 20% more OP effective. Compared to the old 61/50=1.22, or 22% more OP effective.
So it couldn't be a small tweak to hardened shields, it'd need to provide something like 12 effective flux capacitors instead of 6. So instead of a 25% reduction in damage, it'd need to be like a 40% reduction. Not sure what you'd do to hardened subsystems to make it worth 12 OP or so. I fear you'd still run into only a few hullmods getting the hullmod+ treatment, which affect combat the most, assuming you were trying to keep relative power levels the same.
Alternatively, you could simply accept that you are making the base campaign game harder, which would let you make hullmod+ be easier to balance tweaks and might see more variety. Even then, some hullmod+ choices are going to be better than others presumably, which will tend to lead to limited "optimal" selections.
My more general question to the thread is, how different will ships play from currently with fixed OP per story point or other potential changes? And how will that gameplay be better (or worse)? Will final load outs actually be meaningfully different, other than a few OP here and there? If you eventually put all the same mods on, because those are the hullmods which help the ship, it doesn't really matter all that much in how they're paid for in terms of OP and story point accounting.