I actually prefer the horde mode too - or, at least, I think it's more interesting to have that as a starting point, rather than having another player controlling one of your party members in the singleplayer.
A Baldur's Gate-style experience also isn't very conducive to monetisation (since it just sounds like part of the main campaign?) and I think there's a bigger potential for the content to feel tired, if there's only a limited number of missions or campaigns or whatever. At least something based almost totally on combat can feel fresh if they keep adding new maps, new enemies, new items etc.
(That's not to say I wouldn't want to see story elements or "things other than combat" in a horde mode, because I think that was probably ME3 MP's biggest weak point.)
Same here.
While a co-op campaign does sound amazing (and something I would defiantly try out) I don't know if playing second fiddle to the host as a 'Not-Inquisitor" or pre-set companion would really be worth more than one play through. Even if the co-op narrative recognized the other players and/or was a separate, co-op specific campaign, you would still run into issues with the either the narrative bogging down to accommodate multiple player options, or for the scripted events, cutscenes, quests, etc. becoming a turn off to other players.
The perfect example of what I am talking about is SWTOR's Flashpoints. After the second or third run though of the dungeon you would have seen all of the possible variances in the co-op narrative, and from that point on, all of the scripted events just become repetitive white noise; an obstacle to overcome so that you and your party can advance to the next group of baddies to kill. Couple this with the relative scarcity of Flashpoint scenarios overall, and the self contained nature of each dungeon's plot, and you had a system (IMO) that looks great on initial play throughs, but quickly becomes one of the lack luster features of the game.
Now granted, a horde mode is repetitive as well, but only in terms of combat. With these survival game modes, you have a sandbox approach to playing with others versus the scripted approach that a story focused mode (like SWTOR's Flashpoints). Plus you have more options available to play with in a horde mode. Like ElitePincone mentioned, ME 3's MP saw the introduction of new weapons, powers, items, maps, and classes to utilize; each one adding to the replayability of the simple horde mode formula. Things like, additional races (Werewolves, Sylvans, etc.) new spells, talents, or specializations, or new armor and weapon sets will help keep the gameplay fresh and engaging for DA:I.
Now while I would love to see such options brought to a co-op campaign, I don't really see the feasibility in the game spending three hours of narrative exposition explaining why a group comprised of a Circle Human Mage, a Qunari Templar, Dwarven Iron Golem and Brecilian Forest Werewolf are in a party together and how such a thing is possible. Let alone how unrealistic it would be to have the writers, and coders include reactivity throughout the campaign for the various party makeups. A horde mode is rather simple, but it has the flexibility to allow for BioWare to go: "The lore allows the possibility for things like Sylvans and Awakened Darkspawn to work alongside the Humans, Elves, Dwarves, and Qunari, so enjoy your new MP racial options."