I must admit I'm having difficulty imagining what kind of objectives we could have in a similarly small map for DA:I. Close Fade portals and kill mob officers are pretty much a given. Escort doesn't really make a whole lot of sense for the setting unless the maps are significantly larger.
The reason I fixate on combat for such a game style however is because DA:I's core mechanics don't seem to be twitch based(like ME3's were). Twitch based combat can keep a player actively involved even with relatively simple enemy types.
But say the combat is closer to a trinity MMO, spamming a heal/dps/tank rotation against endless waves of trash mobs is not my idea of fun. In my opinion, that kind of thing only really works with complex mob behaviors and have limited repetition before boredom.
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.