To be clear, you are not proposing a change to
list syntax.  You are proposing that a newline
cease being a command terminator in Tcl
script syntax.  The remaining option would be
a semi-colon, I suppose, but I can offer little
hope this bridge would ever be crossed.

The good news is you can already have almost
what you want without changing any of Tcl's
syntax rules at all:

set big_data_strucutre [dict {*}"
                          key1 value1
                          key2 value2
                          key3 [list item1 item2 item3]
                          key4 value4
                          key5 [list item1 [list item21 item22]]
                          key6 value6"]