The obvious and easier (none of this stuff is easy) to do fixes on the Ewoks are the pebble eyes on the masks.
Many of the masks didn't fit well or shifted during the performance leading to the actors eyes clearly being visable behind the more basic masks.
There seemed to have been levels of quality in the mask production with higher quality costume/makeup/masks like Teebo and Wicket and lower quality ones (presumably meant to be in the background and less visable). It's quite common practise do this sort of thing in flims (some of the apes in the Planet Of The Apes films and the zombies in Romero's films were just simple masks but put in the background were the eye would be drawn to more expensive and realistic versions).
In ROTJ however they seem to have been all thrown in together with the higher and lower quality suits rubbing shoulders with each other which spoils the illusion.
I'd imagine that doing some of the manipulations that Ady did to the Cantina patrons would be almost impossible to do on the Ewoks because of the fur.
The eyes, noses and lips are the only places where a bit of animation would be possible.
Sticking some nasty teeth (not quite as nasty as Ganamae's mockups) might be doable in some key areas but really the best way to beef up the Ewoks cred is to build up their culture.
Their voices seem to be a bit uniformly ethnically cute, I wouldn't throw their language out of the window completely because it does sound like a language but perhaps adding a few more harsh sounds in there could create the impression of something less comedic and more warrior like.
Removing the excessively silly noises would help too.
In an ideal world they would all be more like Teebo in terms of uncanny valleyness.
He looks more humanoid but alien enough to be believable and a little scary, the teddy bear look is a problem I'm not sure I can see a way around beyond what has already been suggested.
The bit where he thrusts a spear at Han could be enhanced by editing his first and second attempts together so he makes one decisive jab that Han catches just in the nick of time.
