Q.  Since Total Kitchen Remodeling Is Not An Option ...

Would lighting upgrades/additions be a good investment and feasible in a 20-year-old kitchen?

A.  Absolutely!  Those 20-year-old kitchens usually had only one or two light sources.  Consider adding under-cabinet lighting  for your work spaces, and recessed can lights and/or pendant lights over bars and work areas.

Another option is to use recessed cans with one nice pendant fixure over an island where you might splurge a little to make a big visual impact.  

To provide more functionality, use dimmers.  Dimmers will allow you to customize your light levels based on what lighting is required for the task at hand.  

An added benefit of dimmers is that variations in lighting can change the whole look and mood of any space.