Put as much effort as being an RPG as being a shooter. This game was supposed to be a fucking hybrid, not a gears clone. Shooter elements are fine, just dont fucking saturate the game with them.
Also, take the aforemention RPG and Shooter aspects, and place them below story and character interaction in terms of priority. One of ME2s major flaws was the amount of time spent doing what felt like excuses to shoot shit constantly. "Squadmate Y would like to see you, unload their daddy issues on you, and have them resolved by killing wave after wave of enemy spam".
So yeah, dont just fucking "shooterise" ME3, concentrate on the RPG as much as the shooter, and for gods sake focus on the supposed strengths of Bioware: story and character writing. ME2 advanced piss all in terms of the trilogys main plot, so I want to see ME3 spend the lions share of the effort on a epic, story and dialogue driven send off.
Oh and bring the ME1 LIs back as squadmates. Also focus on less overall squadmates with more in depth content. The lack of general dialogue, squad banter and interjections in ME2 was painful.
Edit: Oh and stop with the stupid "shooter levels" breakdown of the gameworld, coupled with immersion breaking loading screens and mission complete screens. They suck. ME1 felt like an amazing, cohesive experience where you were never away from Shepard. ME2 felt like shooter mission stages and it sucked.