Simple and obvious changes that FDevs could implement in a heartbeat:
Increase travel speed in supercruise
Get rid of the stupid 5,4,3,2,1 countdown that makes you wait every time you enter supercruise or hyperspace J (or this could easily be combined with the 20 second 'other' wait that you have while the drive 'spools up')
Make the crappy SRV handle better so it doesn't trip over every single rock, or spin in a circle every time you press above half throttle (p.s. it's actually faster when you travel in reverse - try it). Says it all.
Get rid of stupid solo kill missions that pay 2 million credits for 90-130 kills that I can GUARANTEE YOU that nobody is taking because they take approx. 3 hours to complete in a max engineered end game ship.
Get rid of stupid solo kill PIRATE missions that pay 2 million credits for 60-70 kills because you can never actually find this many pirates from the correct faction to kill in a period of.. oh id say around 4-5 days of playing time.
Make the 3 material traders into one so you don't have to spend half your time roaming around the map every time you want to do even a single engineering upgrade (that's once you've figured out the actual materials you will need from Inara) - takes me around 1 hour each time through tabbing out etc. Pointless.
Get rid of the ridiculous and unachievable 10000 WEEKLY merits grind for powerplay max level (and all levels in between)
Lose the stupid 5 hour in game flying around pointlessly timer for a notoriety of LEVEL ONE!! and all levels above this.
Put a filter in both the missions screen, and outfitting (stored modules) tabs to sort through items.
Make it so that you can engineer items from storage (not just those fitted to your ship).
Increase payouts for community goals missions - have not made a single profit from these due to creds lost via getting killed from griefers while participating. P.s. please don't bother telling me to learn how to fit my ship either.. a PvP build is not the same as a PVE build.
Bring in a medium sized ship with decent cargohold (think python is currently one of the best for this.. but tops out at around 260tonnes). I'm thinking a medium hauler or passenger ship with more like 500 of space..
I have to run now but this is the obvious stuff that comes to mind without giving it too much thought. Will come back and edit it later to add to my list.
N.b. before you ask - the reason for the fascicious tone to my response is that all this stuff seems as obvious to me as a slap with a red mullet, so if Fdev actually cared about their player base most this stuff woulda been changed already. i.e. not holding my breath. Kudos to OP for trying though.