Just an idea i've had to expand the variety of quest objectives
How about an objective called something like "CustomObjective" where all it consists of is a string in your questholder such as "Tame the White Stag". It isn't dependent on the players actions directly, such as killing a mob or collecting a specific item, but rather spawner based
For example
You could spawn a deer using the triggeroncarried command looking for the questholder. Another spawn would then check to see if the deer has been tamed or not... and when the deer is tamed it would set the objective completed=true using something like SETONCARRIED,Tame the Deer,questholder/completed1/true...
Just a thought... use it/don't use it