Numerical data with lots of NULLs: 6NF or document database?
JSONB might be a good option. If the TYPES of your flags and sensors are always the same (let's say they are all floats and ints), you could also use another strategy:

CREATE TABLE tb
    (id           serial primary key, 
     timetag      timestamp default now(),
     sensor_nr    integer,
     sensor_value float,
     flag_nr      integer,
     flag_value   integer
    ) ;


If you have real values for sensors 2 and 1000, you would then do:

INSERT INTO 
    tb 
    (sensor_nr, sensor_value) 
VALUES 
    (2, 123.456), 
    (1000, 0.123) ;


Or, if there are flags:

INSERT INTO 
    tb 
    (sensor_nr, sensor_value, flag_nr, flag_value)
VALUES
    (1000, 123.456, 1000, 0),
    (2, 234.567, 2, 1) ;


The NaN value would be represented by NULL; and the "no reading" would be represented by just the non-existence of the time/sensor_nr row.

If you need frequent range SELECTs of different types, you would index by timetag; and also by sensor_nr and sensor_value... The insert costs would be relatively high, but the SELECTs could be fast.

If the "new unexpected fields that pop at any type" are of different types (i.e.: you have some co-ordinate pairs (float,float) and not just simple floats) this approach won't be flexible enough. In that occasion, probably JSON(B) and the new indices is probably your best alternative; at the cost of losing (some) type safety.