I don't like the idea of negative zeal because it seems so artificial and visually unintuitive, but increased AoE suppression is something I'll perhaps play around with.
I think biggest problem is all the yellow cover in the mid to late game caused by all the indirect fire combined with RA bonuses with vet. Makes it impossible for hmgs to even suppress squads much less pin them. I think yellow cover shouldn't be effective if the unit is moving through it. It would allow hmgs to keep infantry at a distance instead of letting them too up to then and wipe the gunner.