I've been thinking about Unnatural Characteristics. I've always liked the change between Deathwatch and Black Crusade, where they become additive instead of multiplicative, but I don't think that change went far enough.
When we did the Only War Beta, folks complained that the Unnatural Strength and Toughness for Ogryn was too low. After all, tabletop Ogryn are stronger and tougher than Space Marines but Space Marines had Unnaturals of 4 compared to Ogryn's Unnaturals of 2. It occurred to me shortly after the playtest closed that the problem is not the Ogryns. The problem is the Space Marines.
I have playtested a Deathwatch adventures with Space Marines receiving Unnatural Strength and Toughness 2, and not using the "Astartes" grade weapons. I found the Space Marines to preform heroically, neigh invulnerable to small arms (but not completely invulnerable) but still interesting to play.
I would like to propose reducing the "standard" of Unnatural bonuses by about half and doing away with the Astartes grade weapons. This, I have found, mitigates the overpowered nature of certain adversaries (like Spare Marines) while still making them powerful, eliminates the need for two different types of bolter, eliminates the need for the Felling weapon trait (which only seems to exist to negate the overpowered nature of Unnatural Toughness) and still makes for interesting play. I think it actually opens up the possibility if playing Space Marines alongside (very experienced) humans without the need for a lot of special rules.
I also expect this will be a wildly unpopular opinion, but I though I'd toss it out there.