Mostly the 3rd ability "Spidermines" is a pretty useless skill for Engineer.
My suggestion would be to revert the ability into Tinker which would cause a better mana usage and moreover he would regain is push ability.
Now my main idea comes. Before the mainchange of Engineer he had his turret which he could buff with tinker. After the change he lost this ability which made Tinker a bit more useless skill. So why dont you just let tinker buff the current Turrent? I thought about something like when he is tinkering
A: The turret resets the percentage reduce of damage and pushback.
B: The turret shoots 50% faster and has only 5% reduce per shot.
C: A big bullet comes out of the turret which pushes all enemys that get hit by it back and damages + stuns them for a short amount of time.