N1 - Carrie-Ann Moss. An old asari matriarch or Aria's cameo (some DLC videonotes and weapon stuffs).
N2 - Martin Sheen as a cameo for Illusive Man. Some old beacon with a prerecorded message for a side-mission? I would like it.
N3 - Joaquin Phoenix as a some sort of mature 'thinker' guy in a team. Maybe, a lead researcher or something?
N4 - Gerard Butler for a frontline councillor. And if there would be a '300' reference I'd buy it without hesitation ;D And btw this guy can make some interesting insight into frontline dialogues and social behaviour for some characters (considering his efforts for the 'Law-Abiding Citizen' movie)
N5 - Keanu Reeves for some sort of alien Japanese-like warrior leader. Some idealist alien faction with a knack for philosophy. Could kill the word budget alone though, but it's worth it.
N6 - Kevin Spacey for some cool guy to be a semi-friendly counterpart for the protagonist. You know, someone challenging player at some point for leadership, someone that cool to have a chance to win this leadership.
N7 - Allegra Clark for a secretary. This would be magnificent to say the least!
N8 - Brian Bloom for a serious squadmate this time with a little knack for sardonic humour.
N9 - Kim Bubbs for a computerized suit support, like EDI was in ME2-3, and, probably, AI on board of the flagship.
N10 - Moon Dailly for a crew member, better if a support person like chief medic. One that can insist on what's important. Softly.
N11 - Beata Harju for a combat reporter or a support crew member. Heard her once on finnish radio and totally loved.
N12 - Mirjami Heikkinen for a settlement leader, or some secondary base chief. Strong, recognizable voice with a scent of leadership.
N13 - Kazumi Evans + Madeline Merlo + Shylo Sharity for a band in some club. You'll get an audience thanks to that alone, trust me 
N14 - Jonathan Groff for some kind of a clumsy yet sympathetic guy around. Somewhat close to Alister from Dragon Age: Origins, I think.
Not a complete list, ofc, but I think you get the idea.