I wish you could turn an engine on or off WITHOUT having to select all engines for multi-engine planes. When shutting 1 engine down you have to then select the other engine before inputs kick in, and if you select "All engines" it restarts the shut-down engine.
This is especially annoying on 4-engine planes (as you can NOT select all 3 remaining engines after this point, nor can you select just 2 engines). However, it is also annoying on 2-engine planes.

I wish there were, in addition to "start all engines" a separate key for "Toggle eng1 on/off," "Toggle eng2 on/off," and so forth.
If need be, leave them unmapped by default (like the mode switch keys), but it would be most helpful with engine and damage control in this game!