One Non-Clustered Index that covers every possible combination of predicates?
Multiple aspects needs to be considered when creating an index. An index on columns A,B is not the same as an index on B,A. the performance of a query which filters on both column A and B will be different depending on the order of the columns in the index. 

Indexes slow down DML operations and increase the size of the database.

You can refer to the stackoverflow link below for more details.
https://stackoverflow.com/questions/5447987/why-cant-i-simply-add-an-index-that-includes-all-columns