You've been tasked to solve these questions! How would you try to address them? Breaking the problem down is usually a big help, and you've already started to do that by asking various questions. What are your answers to them? List some pros and cons for each of your answers, and I will do the same.
Alright, fair enough. Let's begin!
How will you handle the party-driven experience of the DA games? Will it require other human players to don support roles?
I will tackle this one first.
If MP is definitely not co-op, then it will definitely not be party based. Losing the ability to pause-and-play, coupled with the resource/server limitations of having dozens of player-controlled characters on screen if everyone gets a party of four is nearly insurmountable to properly address. So, it would need to be individually controlled players.
So, in light of that, you will need to keep support roles, otherwise you slant the overall mechanics to constant, twitch combat. This means that you would, instead, need to focus on MP gameplay not directly tied to combat. There must be objectives to be accomplished that are more complicate than "survive X number of waves." Warrior classes should be wading out in the middle of the action, while support roles are in protected areas, guiding the flow of the encounter, while rogue classes are either performing long range attacks through archers or infiltrating areas stealthily to reach said objectives, or possibly laying traps, so that enemy characters entering areas where objectives are would be blocked/harmed. Essentially, just like non-combat skills and class-specifc skills are strongly valued in an RPG SP experience, they would have a similar ability to enrich the MP.
There should be a good player matching system for sessions, where players who choose to make builds that are not classic DPS max/min-ers are the only ones joining up and the only ones other players needs. A healer/buff Mage that could know how to protect themselves and control the flow of the battle would be an invaluable, while a rogue that can actually steal the Golden Snitch (or the objective equivalent) would be similarly indispensable.
How will you handle the party-driven experience of the DA games? Will it require other human players to don support roles? If so, I can see problems with that. Who would want to build a character that relies on other characters (outside of their control) being built right? In an MMO, it is easier, since you can Look For Group or even be part of a guild. With a MP component, it is far trickier to just hop in a match if you can't count on the right support skills.
Also, a large portion of the draw of MP is the action. Sniping enemies,
throw grenades, using biotics... how will that work in a game like DA,
whose auto-attack requires a lot of just standing and swinging? It is
not like you control an archer and aim their bow... you just target the
enemy and hit A. Same for every other class. Unless you are using a
skill or kiting, there is not much else going on for control over one
character at a time.
In terms of this, I use the same answer as a above - make the MP sections not about (solely) combat. If you do this, and give rogues a task to sneak of scout, or give Mages a spell form to disrupt, etc., this could provide an incentive to make more diverse builds, not focus on just DPS-maximization and gives very different gameplay feels based on class and/or setup. It also avoids the "everyone uses medigel to revive and other "one-size-fits-all" types of gameplay mechanics. In addition, one does not need to "jazz-up" things like the Auto-Attack or the pace of combat, as you can instead have combat focused builds, non-combat builds, versatile builds that can handle both, etc.
Slide in some unlockable kits that allow for different builds, multi-classing, different races, different equipment types, various level up/bonuses and suddenly you have a viable MP system that can also result in revenue generation. You can use various plot setups to do this, like great battles of the past, Mage vs. Templar, Arena, Darkspawn Hordes, etc. (all suggested by others in this thread) and it can deliver a decent enough story. However...
Again... not impossible things to do, but it very quickly becomes a different game than what the DA series has been.
This is the concern. The MP becomes, in nearly all aspects, a different game. And where does the line begin between "making the MP and SP components more similar to share alike resources" and "turning the entire experience into something that is not DA?"
I'm not saying it is impossible to do a MP component... I am simply saying it is not wise to do so. It will turn the series into something fundamental different, something it is not. Though I do appreciate the excuse to do some armchair development, fixing the problem presented isn't impossible... it is seeing the current situation as a problem at all that is what may prove to her harmful.