I don't think it was possible to make a good D&D movie in the 80s, for a very simple reason: the people making the films at that time hadn't grown up playing D&D. In order to make a movie that is anything approaching "good" the film-maker needs to have a personal and emotional investment in the source material.
This is a very interesting point.
I think there's a bit more to it than that, and that you don't have to grow up with something to be invested in it, many directors care deeply about scripts new to them, for example. But I do think it is unlikely that Boorman would have been deeply emotionally involved in any D&D movie which actually, er, resembled D&D in any form.
Imagining a D&D movie c1980 that has an impact similar to 1982's
Conan the Barbarian. Regardless of why you think that movie had the impact it did, it inspired or co-inspired a wave of '80s fantasy movies. The main effect I see is - assuming this D&D movie has one or more elves, dwarves, and whatever else - is more non-"human" characters in those '80s fantasy films. Maybe screen-worthy prosthetic ear technology takes a moderate step forward.
The D&D movie would be one of those movies, though, it couldn't really have had the impact Conan did because of that. The main effort to get it made was in 1984, and if it came out in, say, 1985, it would have been up against
Back to the Future,
The Goonies,
Breakfast Club,
Rambo 2,
Rocky IV,
Weird Science,
Fright Night,
Commando,
Mad Max 2 and so on (wow 1985 pretty classic year huh?). Look at these movies and compare them to what Boorman actually came out with in 1985 -
The Emerald Forest - he's clearly got slightly more action-oriented, but it's more like a dubious late-70s movie than anything from that era.
One thing we can be pretty sure of though - he'd probably have also cast his son in the lead of the D&D movie, like he did with The Emerald Forest (his son is actually in many of his movies, just not as a lead) - he certainly looks like he could play Elf/Half-Elf.
With a different director and writer, and maybe coming out slightly later, I could see a D&D movie having an actual impact, so long as it was very much part of that '80s wave of movies - somewhat self-aware, audience-aware, charming/witty and so on, not the self-serious movies of the earlier era that Boorman is part of.