Allowing multi-line and indentation would dramatically increase readability in Timelion expressions.
The way I'd love to see is the following:

(
    .es(timefield=timestamp, metric=cardinality:somefield.keyword)
    .label("Nice Label")
    .bars()
    .yaxis(yaxis=1,label="First Axis"),

    .es(timefield=timestamp, metric=sum:anotherfield)
    .divide(12)
    .label("Another Lable")
    .yaxis(yaxis=2, label="Secondary Axis")
)
.title("Graph Title")
.legend("ne")

Also, when editing a timeline visualization in Kibana, the left panel is too small and I'm forced to edit the expression in another editor and paste there. Maybe allowing the user to resize the split Editor/Visualization is ideal.

Syntax highlighting would also be very welcome