I think that these are changes just about every engineer will agree to and are not controversial:
Box of Nails - Reduce cast time from 0.75s to 0.25s. Increase bleed and cripple duration from 2s to 3s.
Why? This skill is and has always been used for slowing an opponent down while kiting around the environment. A 0.75s cast time is quite long and leaves you pretty vulnerable for something that just cripples. The increased duration is because so many things exist in game that practically make the cripple a duration of 0s.
Pry Bar - Increase damage by 10%.
Why? We lost our 10% damage boost trait a while back and pry bar does not even hit that hard compared to many other skills in the game. Pry bar is one of the very few "burst" skills that engineer has other than grenade barrage.
Grenade Kit - Make the velocity and radius increase baseline.
Why? Grenade kit has seen many nerfs over the years and is practically never used competitively anymore because you're forced into explosives for the trait. Even with the trait, it's generally just taken for grenade barrage. The kit is almost entirely useless in large fights because of projectile defenses. Explosives itself is bad and needs an overhaul like warriors tactics traitline received.