Timeline of plays

As I was doing some background research on Richard II, I was looking at the date it was written.  The following excerpt from a chart I found with the left column showing the year in which these plays were first performed and the right column showing the year these plays were first published (as well as we can tell).

1594-95	Two Gentlemen of Verona	1623
1594-95	Love's Labour's Lost	1598?
1594-95	Romeo and Juliet	1597
1595-96	Richard II	1597
1595-96	A Midsummer Night's Dream	1600
1596-97	King John	1623
1596-97	The Merchant of Venice	1600

What I find very interesting is that at the same time that Shakespeare was writing Richard II, he was also working on Romeo and Juliet and A Midsummer Night's Dream.  What does that mean?  I don't know, but it is interesting to think that the themes and characters of these plays are all bouncing around inside Shakespeare's head at the same time.

As Shakespeare writes Richard II - with it's warring factions, Bolingbroke against Richard, did he dream up the idea of the forbidden love that makes Romeo and Juliet the classic that it is?  

One thing to consider is the staging of these plays -- Any theatre will tell you that you have to have variety in your repertoire, you cannot perform only tragedies or only comedies.  So maybe the fact that these plays were performed in the same year is only a reflection of the practicality of running a theatre as a business.  

But still, it begs the question, are there common themes that cross these plays?  Is Shakespeare dealing with similar issues in these plays?