Heroes replacement at level 40+

Chewbacca is very useful throughout the levels due to his taunt and can as are result protect your weaker characters if you are waiting for a heal to come off cool down... Although you may want to replace him with Han later for the turn meter removal he has especially if you are well covered on heals.

Like others have said Luminara you definately want... when her abilities are significantly upgraded, her raw heal rivals that of Barris along with a 10% heal every turn for everyone... Although she doesn't have as much health, she does more damage and also has a very useful ability lock attack.

Notable characters to look for is Obi-wan, not used him but faced him a couple of times and he is very irritating to fight against probably more so than Douku

I would look to replace the Jawa with either Kit Fisto or Aayla Secura...

Ahsoka I would only recommend if you are low on healers as damage output wise there are better out there, but she can pretty much keep herself alive without a healer. And she isn't as versatile as other attackers in applying other effects on enemies.