They could have all sorts, but as you said collecting stuff will probably make up at least one if not a few achievements. Seeing as collecting hidden packages in the old games gave you weapons at intervals of around 10, may there will be achievements for finding one, half and then all of the packages.
Story completion ones will probably make up a few, side missions and that sort of thing may have their own. I'd like to see a 0 pointer for activating a cheat, and to stop people using cheats to beat the game maybe they deactivate cut scenes and the cut scenes activate the achievements. Therefore cheats = no cutscenes = no achievements.
There is so much potential for crazy achievements, they had better not crap out and put in 5 for pointless things.