Awesome breakdown of a DB trilogy,
Kaboom 
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As someone else mentioned, I agree that DB just seems so different from the sci-fi martial arts thing that's DBZ, that the only way I could really think of combining the two is having something like what Snail said about having the Budoukai get interrupted by Saiyan ships and then going on from there.
Kaboom, your little re-write of having the entire Earth get prepared for the Saiyans coming to invade is pretty interesting. I can see Bulma in that scene, passing out some kind of extra-strength guns to all the non-fighters in the crowd, made by her.
As I continued reading your little synopsis, I thought that having Krillin stay on Namek only to have him die by Freeza is kinda well... lame. I know that that's pretty much how the manga had it, but I was thinking of him being at least being a bit more helpful.
I was thinking of Krillin seeing Piccolo's regenerative powers in the first movie, and noticing that Freeza doesn't have such a power when he throws a Kienzan at him, only to have Freeza dodge it, with just his tail removed. After figuring that out, he tries to tell Goku as they (maybe Vegeta and Piccolo included, unless they've already been killed) continue to fight Freeza, but unlike the manga, nobody really has enough time between throwing and dodging punches to talk to anyone. The fact that Goku's not exactly the brightest light bulb out there also adds to the audience thinking "Augh, if only Goku was smart enough to figure it out on his own!"
It's not until Goku finally puts two and two together that he finally gets what Krillin was trying to tell him about Freeza. But it's right at that point that Freeza lifts Krillin into the air and explodes him. Like in the manga, this triggers Goku's SSJ transformation. A little re-write/sidenote I'd like to add here is that 1) a significant point is made preceding the SSJ transformation that it takes time to learn an actual technique (you can't just look at some new kind of ki-blast and copy it right then and there) and 2) the SSJ transformation, being the uber power-up it is, allows the person to be able to copy another person's move right off the bat. Upon reaching SSJ, Goku takes Krillin's move, the Kienzan, and uses it to slice Freeza to bits. If there's time, there can also be the whole thing Goku loaning his energy to Freeza and Freeza using it against him, yadda yadda, but I think, the scene can stand well on its own right there.
I never really liked how Krillin was merely used like a stepping stone that Goku used to reach SSJ, so I think this "re-write" would be good for Krillin's character.
Building off of what
Big Momma said, I was thinking of Goku stil being in that healing chamber, and suddenly breaking out of it, as he sensed that Vegeta and Piccolo died, leaving poor ol' Krillin by his lonesome.
You also brought up that in your version, Namek would be a special planet that couldn't be reached because of some kind of something. Does this mean that once Freeza broke this something that anyone else would be able to reach the planet (ie: our heroes) as well? 'Cuz if not, and the heroes would have to reach the planet through some kind of portal device thing (maybe made by Bulma) that could transport someone to any place (maybe looking like a watch in the sense that you wear it on your wrist and so, each person has to wear one, since they can only transport one person) then I was thinking of having those devices be destroyed during the fight with Freeza (hehe, maybe putting them on your wrists wasn't a good idea after all) just to make it seem that much more impossible for Goku to be able to escape.
The bit with having one of Bardock's team survive planet Vegeta's explosion was also a pretty nice touch, as Goku never really learns that his father played such a significant role in his race's past, huh? Also, the idea of having the survivors from the Saiyan invasion live on a "drifting colony fleet" just has me picturing the characters living in some kind of space station a la
Xenon: Girl of the 21st Century. It's still a neat idea, though, and as you mentioned is the perfect opportunity for cameos. Having Trunks there to give the downtrodden heroes the hope they needed would be a pretty nice touch, too (even though that might just add to the already confusing time-travel in the series...). You could even hint that it's Trunks post Cell/Androids being killed, meaning that he definitely would be pretty hopeful of stuff eventually ending up for the better in the future.
There could also be some little references to the more hardcore fans out there. For example, if there would be a scene where Piccolo and Nail fuse, there could be some kind of series of dramatic camera changes, leading the fans to start to expect Piccolo spewing his infamous "YES, YES, YES" dub line... only to have him quietly say something like "Wow" or "Holy Sh*t." As long as they don't include that horrendous "Over 9000" reference in the movie, I think I'll be fine.
Eh, it seems like I'm just fleshing out what you guys are saying, since I've just got too much work on my hands to make my own little synopsis from scratch. Oh well

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