What will future populations of stars be like? As stars slowly using up hydrogen and ejecting heavier elements into space, the future stars will form from nebulae that are more metal and helium rich.

The question is: how does the increased ratio of helium and metals affect star formation and stellar evolution?

Does more helium and metals in the core at the beginning means the star begins it's life in an advanced state so it dies faster?
(Adams &amp; Laughlin 1997) discuss the effect of increasing metallicity in the future. A higher metallicity increases the stellar burn rate since the density increases but 
the higher opacity reduces it a bit. The total effect is nonlinear; in their model the maximal lifespan happens for $Z\approx 0.04$ and beyond that it declines. Whether there are newer and better models of this I do not know.

They also point out that the maximum stable stellar mass decreases as metallicity increases, while the mass of the minimum mass mainsequence star declines, both as a result of opacity effects.

Overall, the metallicity growth is likely to continue across the stelliferous era. While extragalactic gas may fall in, it merely adds to the hydrogen pool and continued star formation. What could stop the metallicity growth is quenching due to a galactic merger or active galactic nucleus that blows away enough gas to stop further star formation.