How does pH depend on temperature, when taking into account volume expansion?
Your qualitative understanding is correct.  As you increase the temperature, you have two effects:


The dissociation reaction, which is endothermic, shifts to the right.  This effect increases the concentration of hydrogen ions. 
The concentration of hydrogen ions is expressed as molarity, which is number per unit volume.  As you increase $T$, water expands, resulting (everything else being equal) in all concentraitons decreasing.


And you're asking which one wins. 

The answer is the first, overwhelmingly.  For instance, as you go from $\pu{25 ^\circ C}$ to $\pu{50 ^\circ C}$, $K_\mathrm w$ increases by five times.  By contrast, the increase in volume over this temperature range is only $1\%$.