chaining brews is totally legit, and don't even worry about making it a 1-gallon batch.
The rule-of-thumb is that you chain brews from lower-to-higher gravity, lower-to-higher hops, and lighter to darker styles.
So I would do a nice, quaffable pillsner or patersbier, and harvest that yeast into my next IPA/DIPA or Trippel or Stout. For most liquid yeast you can brew a full 5-gal batch without an additional starter for OG < 1.050. (I'm personally a fan of 3-gallon)
It's also a great way to play with different expressions of the same yeast. a while back I did a light SMASH with pilsner and Citra using one of the Belgian strains. (forget the specific one) kept the temperature low throughout, and it produced a light fruity beer, almost a shandy really. Then pitched that same yeast into a triple and let the temps go a little crazy. That one got coriander and clove and pepper, which were all completely absent in the original. Have fun.