There is not enough budget to make the special effects bo on par with A New Hope. So: Don't even try.
Instead, make it a more quite, character driven film. Something that could work equally well as a little stage play if anybody wanted to do that. This will of cause break with the Star Wars franchise being action & space opera movies. But: It is Christmas and if well done the producers may get away with it. And some people may right away see that Star Wars can be a lot of different things, including something more quiet. Of course lots of people will be irritated ("This is Star Wars?") but when later The Empire Strikes Back hits the cinema things will be back on track and the Christmas Special latest by then will have established itself as an exception to the 'rules' of Star Wars. And (as per OP's request) a good one.
With that as the basic concept ('skip space opera and special effects for a brief while and focus on character backgrounds, development and setting up episode V') the show could go in a lot of direction as to what it will be about. My best nebulous first ideas would be:
After the battle of Yavin, the Rebels are again in dire straits and under pursuit (which nicely fits in between A New Hope and Empire Strikes Back). The main characters arrive at a temporary outpost which dialogue (lets say with a Rebel extra already there) reveals is probably already exposed and will need to be evacuated again soon. A new base is supposed to be set up on the planet Hoth the rumors go. But no one knows for sure how things are supposed to continue for the rebellion. The whole feel of the setting is fleeting. An underground base (i.e. allows all scenes be shot indoors) with rebel personnel in a state of general disarray getting a brief respite. But the gathering of a new storm is already palpable.
But: There is a brief respite. A calm. And during this lull between 'episode IV and V' so to say we can have a play acted by the main characters how they deal with the recent events of episode IV. Kind of an elaboration of elements that were left a bit unspoken between the two movies. E.g.: Han Solos decision to stay with the rebels in spite of Jabba's bounty still on his head. How Leia copes and rallies after the destruction of Alderan. How Luke deals with his confusion about what he seems to be able to do with the force. Some mention of how Chewbacca ended up with Solo and his attachment. Etc. etc.. The Christmas Special perhaps does not even 'resolve' any of these issues the characters sruggle with. It just presents them (so future films can lean on that) and instead ties up towards the end with the characters in spite of all their woes rallying each other and confirm their commitment to each other and the general cause of the rebellion. Overall such an outcome would I think fit very well with the Christmas season.
In summary: Use the Christmas Season for a little bit character development in between the big movies, make it less flashy and more quiet and have its 'message' tie in with the Christmas season ('unity, friendship, peace in space and on earth ... some day').