Check out the Kawasaki Vulcan 500. Decent power and comfortable. Also about the same size as your GS400. But if you can keep the GS400 going I would definitely do that for as long as possible. It is a better bike than anything made today.
2002 Vulcan 750, 2001 XT225
2008 Vino 125, 2009 Genuine Stella
2012 Zuma 125, 1980 Puch Newport moped