Show window preview when switching windows of the SAME app
F11  will show all current application windows. left/right arrows will step through them. Enter will select.
This is a subset of Shortcut for toggling between different windows of same app? which you can invoke similarly from the  Cmd ⌘   Tab ⇥   app switcher, except in that case you use the down arrow to switch you into this mode.
If you'd find  Cmd ⌘   ↓   better all round, you can change that in System Prefs &gt; Keyboard &gt; Shortcuts &gt; Mission Control - though you can't do the inverse &amp; make both  F11 
If your habit is to always go right for  Cmd ⌘   Tab ⇥   every time, then you can quickly do left, down to flip through windows in the existing app.
You call as to which you might prefer.