I do not believe 20 attachment points is stronger than the current signature weapon bonuses.
What attachment could the increment of 20 attachment points (after you have already spent 100 attachment points on a weapon and have already equipped most high-priority attachments) afford you that would yield a stronger bonus than any of the current signature weapon bonuses?
In most cases this additional 20 attachment points would only yield a laser sight (which is basically the Engineer's current signature bonus with the added downside of having a visible laser) or an ammo type, or a small upgrade to your foregrip (resulting in only a very small increase to recoil control).
20 attachment points IS a minor bonus. This change primarily gives the player more flexibility in choosing how the signature weapon bonus can benefit them and does not upset the current balance of power.
One could even make the argument that the 20 attachment points I am proposing is actually weaker than the current signature weapon bonuses on account of Recon's signature bonus increasing their fire rate, as there is currently no attachment that accomplishes this. This could be easily solved with the addition of a "quick bolt" weapon attachment, however.
You might be right. It is just a guess from my side.. I admit that. I have obviously not tried all the weapons (other than the weapons available in the beta) and seen all the different builds possible with the attachments, so I don't know. I also think this would be even harder to balance? as it would (at least based on the available attachments during the beta) seem to benefit certain weapons more than others. Hard to tell without actually testing builds with 120 attachment points though.
Having had the chance to play around with the full list of attachments, I definitely do not think the 20 additional attachment points would be overpowered. On most of my builds, I would dedicate the additional points to an upgrade to the foregrip which would yield only an improvement to ADS time or invest in a different ammo type. On my snipers, it would afford me a bipod and the cheapest laser sight.
Regarding your point about the difficulty in balancing this proposal: one of the best things about this suggestion is that it is based completely off of the attachment point system, which requires balancing anyway. In terms of balancing the additional attachment points given by the signature weapon bonus, that would just be a matter of changing the number. If 20 is too much, it could easily be adjusted up or down.
