Well, you can't learn something that no one has created because there's nothing to work from. If you want to create something totally unique, you can (as you said) tweak random knobs for hours/days/weeks/months on end to finally get something you like. The disadvantage in this is that you won't fully understand how or (more importantly) WHY your configuration of Massive made that sound. Your best bet is to just read the manual. Read it thoroughly. Twice. Three times. Make educated notes. Start over and make notes of your notes. Google terms you don't understand. Google terms you do understand to make sure you're correct. Experiment with your knowledge. Use waveforms and effects you don't normally use. Make sure you know what each filter does and where they can be applied to get a particular sounds you're after.
At the end of the day, it's the time and effort you put into your work that will bring progress. Experience is what you need more than anything. You could watch every single sound production video specific to Massive in existence, but it will not bring you any closer to where you want to be. Invest bucket (no, cargo ship) loads of time into the areas you want to improve on. It's the only way you're going to "move forward".
Regarding your other points, again it's the time you spend in the studio that will make progress. A good idea to start you off in the right direction is to listen to some of your favourite tracks (preferably in the genre you make music, but not essential). Study it. Dissect it. Write down every single sound you hear in that track, where they're introduced, when they're accentuated, etc. This will help you fill up your tracks and make them sound full. Use these tracks as templates for your own. Follow the flow of the track, but don't be afraid to let your creativity take you off in a different direction.
If you want to improve on your melodies and harmonies, study music theory and apply it to your work. Here are some links to get you started:
http://www.musictheory.net/lessonshttp://audio.tutsplus.com/category/tuto ... mposition/http://audio.tutsplus.com/category/tuto ... ic-theory/I've kind of lost my train of thought right now, but my main point in this post is that you HAVE to put in the hours to see progress. Sometimes it's hard, sometimes you feel uninspired. But that's no excuse to work on other areas of your music. There are too many areas of music production for you to NOT have something to improve in ANY circumstances. Make the most of your time. Work hard and work SMART.
Extra link that may be helpful
http://audio.tutsplus.com/category/tuto ... nd-design/DISCLAIMER: This post may be plagued with spelling errors and silly ideas that some silly 18 year old goes by.