I think the problem in this thread is that many are trying to find a way to:
1) Start the film series in the middle of the story (Saiyan Saga), instead of starting from the very beginning.
2) Make the films realistic and darker.
3) Adapting DBZ's plot literaly, almost scene for scene.
I think there are 2 major problems with this, Dragon Ball is, has always been, and will always be colorful, some of these attempts to make the franchise more "grounded" are only making it look more generic, i don't think we need Man of Steel 2.0, hell, Dragon Ball Evolution also tried to be more realistic and it was only hurt by that.
Stop thinking about how "extreme" you can make the fight scenes, and just think of Dragon Ball as Wizard of Oz and Star Wars with Martial arts, do not start trying to be part of a trend, create one.
I also disagree with the idea of starting with the Saiyan saga, it opens too many troubles because you're already expected to like these characters, know them and be somewhat familiar with some of the concepts. As a kid you don't realy care about it, but most people who watch DBZ when they're older seem to ask the same kinds of questions, like "why do people in this world all have powers?", another problem with making it the first film is that Goku dies right at the beginning, you've just been introduced to him, and now you see him dying and coming back, and you'e expected to care.
Let's look at the original Dragon Ball concept, you have a classic tale about a group of people who travel in order to find some MacGuffin, but end up finding themselves, while having some cool adventures here and there. That my friends is a classic tale, and you have the strength of an interesting world, unique characters (i don't think there's any live action film with someone quite like Goku), a strong mythology and a lot of potential for visuals. With all this i think it would be a real shame to just skip and go ahead to the alien invasion, i think there's more interesting stuff to cover first.
Now, regarding adapting DBZ literaly, i think the plot of each DBZ saga is actualy easier to tweak around than DB's, the fights were longer, so the plot gets less complex, then you have times when Akira Toriyama was just buying time and seeing which ideas worked or didn't, Majin Buu saga was pretty much like that as a whole, so there were a lot of things that didn't make much sence in there, as the manga returned a bit to its gag roots.
Regarding the wish to make these films dark, it's Dragon Ball, if anything you need to go back to the 80s films tone, the films need to be fun, not pseudo realistic. Even if you want to do it like Harry Potter where each film gets darker...you can't, the last DBZ saga wasn't particularly serious (Majin Buu), quite the opposite, if anything, the scariest bits were in King Picolo saga, which wasn't even from DBZ.
My take is, do Dragon Ball as this entertaining film mixing fantasy with martial arts fights, with a complete story, probably having Red Ribbon as the main villain, then do a 2nd one with Tao Pai Pai, Ten Chin Han, etc, hyping Picolo and probably ending with Goku preparing to be trained by Kami. Then the last film could resolve around Picolo, instead of having been sealed, King Picolo from this universe could have been killed many years ago, so the Picolo in this film would already be Picolo Jr, it would be interesting to have his father's servants trying to convince him to follow on the legacy.
4 and 5 would be Vegeta-Frieza sagas, it's a good place to finish, if a good script for Androids-Cell came up, then you could continue. Either way, i think trying to go right to DBZ is a big mistake, Red Ribbon and King Picolo are stories more worth telling on the big screen than Majin Buu IMAO. (I liked Buu, i just don't think it was the best part of the manga)