A bit generic, but do not neglect looking into the various grenade options you have.
This goes for all classes really.
Some grenade types can really synergize with your build or they can be the tool that gets you out of those situation where your build had shortcomings.
The engineer has some really good choices.

Unlocking the different weapons are also worth it. Some are better for different mission types and others might just suites your play style better regardless.

Rock and Stone