1) Modulation. And make sure that your element fullfil the entire spectrum. Some tracks have few instruments going on but they are so big that it sounds full (Even with only 1 bass 1 kick and 1 snare)
2) Same as 1. I've never tried to duplicate and pan differently. Might worth a go, but I don't get why do you ask about it if you say it sounds great already. Dimension expender can be good, but don't overdo it. Don't forget that exiters at the mastering stage will make everything thicker. You can also try some reverb with a really low wet amount and a really short tail (<500-600 ms) on several of your instruments. Delay could also be a way to add without really adding new things.
3) Mixdown is, to me, at the borderline between techique and art. There is an objective part, cleaning and removing mud and unwanted frequencies of each instrument.
But there is also an artistic (and thus subjective) part. What is the main focus of the track ? The drums ? Or the bass ? Every track is different, and you can mix the same track in thousands of different way for different results. Consider that choosing the volume is a part of writing the track, and no one should dictate you how to do it. Put all faders to 0 and then bring them up 1 by 1 until you find it balanced and you think you put on front what you want your listener to pay attention to the most.
To sum up i'd like to quote someone (I think it's the guy in the video below who said that but I don't remember well)
"a good painter is able to control the path of the viewer's eyes on his painting, and manages to make them look at elements in the way he wanted them to. It should be the same with a good musician".
Levels are not always constant, and you can bring an element on front, then bring it back but BECAUSE it was on front at first, the listener will still remember it and hear it as loud as it was before.
I'd like to give you an example. To demonstrate this let's experiment. There is a track of mine on my signature (and i'm not doing this to get plays, even if it looks alot like it ahahahah), I'd like you listen at the first 20 seconds
before reading the end of my post.
------------------------------------------
It has a piano going on at start. Few seconds later, the level decreases, but you still hear it clearly because I showed it to your ears at first. I wanted it to be the first thing you notice. Then I draw your attention to the synth, and then the drums kick in and you switch focus again. Did it work ?
PS :
You might want to have a look at that :
http://www.youtube.com/watch?feature=pl ... Nfd-APQUfo#!
Thanks to the above person for Mapulator!! Really sorry, I can't get your name up on my phone so I don't mean to be rude 
