Would it be possible to make stackable all parts which have neither resources nor modules?

Action Groups Extended author here. My mod should actually do this already via the KISConfig.cfg moduleManager patch file that resides in the AGExt directory. (The ModuleAGX partModule is mine.)

If this is not working for the ModuleAGX partModule, I would ask you file a bug report for AGX about it.

@SirDiazo I see, then I'll get to the Hot Spot author with your example.

It's already the case. If structural parts do not stack, you must have a mod that adds modules on those parts.
You can add those modules in the StackableModule node in the settings.cfg if needed however.

I see, thanks, from the MM cache I found that every structural part on my install has these modules:

    MODULE
    {
        name = ModuleAGX
    }
    MODULE
    {
        name = TweakScale
        type = free
    }
    MODULE
    {
        name = USI_ModuleRecycleablePart
        Menu = Scrap Part
        ResourceName = Recyclables
        Efficiency = .8
    }
    MODULE
    {
        name = USI_ModuleRecycleablePart
        Menu = Salvage Part
        ResourceName = SpecializedParts
        Efficiency = .4
    }
    MODULE
    {
        name = EnhancedThermalDataModule
    }

The mods which add these modules to every part are: Action Groups Extended, Hot Spot (formerly Enhanced Thermal Data) and obviously TweakScale. Do you think you can include them to the default settings? Probably excluding TweakScale.

@KospY Continuing the issue, what do you think about adding these stock modules to the default list? They look very stackable to me:

    moduleName = ModuleRCS
    moduleName = RetractableLadder  
    moduleName = ModuleDeployableSolarPanel
    moduleName = ModuleDeployableRadiator
    moduleName = ModuleActiveRadiator
    moduleName = ModuleLandingLeg

@enceos : I added your list to the next version of KIS, thanks :)