A translucent top bar and new window maximizing effects will be on show in GNOME Shell 3.26 when released later this year.
GNOME 3.26 will have the top bar translucent by default, giving a more roomy, spacious feeling to the desktop. The top bar will return to a solid colour when a window is maximized against it. This extension fades your top panel to nothingness when there are no maximized windows present. Never again will the panel be abruptly darkened!
You don’t have to wait for GNOME 3.26 to get a transparent GNOME panel though. There are various extensions available to do a similar job.Alongside the new top bar stylings are improved window animations when moved to and from ‘maximized and half-tiled states’.