I think more achievements would make sense for a game that has such a large average play time. I love achievements, though I'm not really a fan of a bunch of achievements that are things like completed tutorial, completed map 1, completed act 1, etc. At least make them things that you might have completed during those things... instead of something like completed act 1, have it be something like defeated boss 1. That's of course a general statement for games and not specific to this one, which doesn't have acts or bosses. I prefer achievements that actually require some effort, though. I'm also not a fan of games that have DLC achievements mixed into the base game achievements, preventing someone from getting 100% unless they buy those DLC, or games with PVP achievements that can't be completed without playing PVP. The 25 kills one and the lower ones are fine since they can be done very quickly by someone not interested in PVP by just doing it with a friend, but having something like 1000 kills isn't something I want to see.
AFAIK, games can add/remove/change achievements whenever they want. However, there are a lot of people who for to complete 100% of achievements who get very upset when a game that was completed now is only 50% complete or whatever. So if they add more achievements, I think they should plan on doing it only one time and then leave it. That means making sure they know every achievement they want and add them in a single update (probably the story update since that's supposed to be the full release version).