As the title states, this is for every possible improvement that could be made towards ME3 with the exception of how to move the plot, as there are already several of those blogs out there.
The main flaw of ME2 in my opinion was the absence of mission maps. The only way I could see a map of the mission area was by using the guidebook. Aside from that, only the hub areas (Omega, the Citadel, and the Normandy SR-2) had maps, and even they felt rushed and too crowded. In ME1 there were maps for all levels. A likely improvement should be a map system for all levels, complete with a 3D interface and with all elements in real-time, not showing static positions like in the first two games, similar to the map system used in the Ghost Recon: Advanced Warfighter games.
Feel free to state others you have in mind.