I'd love to see multiplayer campaigns with group dialogue, similar to the group content in The Old Republic.
A survival mode could also be cool, especially if they expand on what was offered in ME3. I liked ME3's multiplayer fine enough, but it felt a bit lacking when compared to something like Gears of War 3's horde mode, which was basically the same thing except you had the ability to erect turrents and wire fences and upgrade them and stuff, which I found fun. Letting you board up doors and windows and create baliste and catapults and stuff to hold off hoards of enemies attacking a keep would be cool. No random loot boxes though. Let us straight up buy the unlocks we want for gold earned ingame.
I think that a 'siege' mode or a 'defend the keep' objective could work wonders at adding variety to the tried and true horde mode formula. Having to juggle not only the team's ability to kill the enemy hordes but also being tactfully aware of defending key locations would help alleviate the repetitive nature of survival game modes.
Being able to use emplacements like catapults, trebuchets, or qunari cannons to try and soften up the approaching armies, to going to the main gate to hold the courtyard when they break through would really help make the MP feel less like a conveniently placed arena that serves no tactical purpose, and more like players are actually defending real-estate in the DA world from encroaching baddies. Something less like a gladiatorial arena, and more like the battle of Helm's Deep.
Plus, what better way to showcase the Frostbite 3 engine's renown for destructible environments; and in DA:I's case: re-buildible environments; then by having the castle become damaged as a result of the fighting; and then having to decide whether the team's mage should rebuild and repair the main gate, or save her magic for the next approaching wave.
Bonus points, if BioWare could have the keep being defended be the host player's version of their SP fortress Skyhold; complete with any upgrades (both cosmetic and tactical) they may have purchased for it; making the battle vary depending on how a host has customized their stronghold in SP. Maybe their Rogue Inquisitor upgraded the Skyhold with numerous traps and hidden passageways, thereby allowing the team to execute ambush and hit and run tactics to whittle down the opposition. Maybe the host has a Mage Inquisitor that decks the castle out in defensive glyphs and shielding spells, allowing the team to ride out the siege in relative comfort while they attack enemies from atop the fortress walls.
Allow the SP to affect the MP instead of the other way around. Maybe the host has sided with the Mages, so the enemies assaulting Skyhold would be comprised of primarily Red Templars. Or the host has lost several crucial keeps and so the Skyhold is damaged at the start of the MP game to reflect that.