alter table or new table : a design puzzle

This is a problem that bothers me whenever there is a need to add a new field to a table. Here the table has got about 1.5 Billion records (partitioned and sharded so it is physically separated files). Now I need to add a nullable field which is varchar(1024), which is going to accept some json strings. Here are the arguments

[]All existing rows will have null values for this field. (fav. new table)
]Only 5% of the newly inserted records will have value for this. (fav. new table )
[*]Most of the current queries on the table will need to access this field. (fav. alter)
Now should I add to current table, or define another table with same primary keys to store this data. Your comments would help a decision. Thanks