Why it had to be Sirius

[mhershey2001]

No: HPFGUIDX 75651

In the weeks leading up to the release of OoP, after JKR revealed 
that a major character died, there was a lot of speculation about who 
it would be, and if I remember correctly, Sirius was not high on 
anyone's list.  Now that we know it was him, and now that we have had 
about six weeks to analyze every single word/theme/hint/prediction in 
the book, I am wondering what people's theories are about why, in the 
name of the plot, Sirius had to die.  There are a lot of 
possibilities, some obvious, and some less so.  For example,
1.  To give Harry another loss of a parent figure, in order to 
encourage some sort of internal emotional growth necessary for his 
development into a person capable of killing LV.  (but you may argue 
that any of a number of other people could have died instead)
2.   To create difficulties for the OoP along the lines mentioned in 
the thread "inheritance" regarding who gets GP, how to deal with 
Kreacher's knowledge, etc.  (this seems to me to be the most pure 
theory for "why sirius" and not someone else)
3.    To create a reason for Harry to travel "beyond the veil" to 
gain some understanding he needs to defeat LV.

Any other ideas?  

Mhershey