Lol, this is just a purposeful limitation the team put in the game. It doesn't mean meters aren't possible at all.
The way they coded Final Smashes in Brawl... well, the Final Smashes each have their own file for each character. So, when Mario does his Final Smash, it calls his Final Smash file. There's a few exceptions to this rule, however, and that lies in Fox/Falco. Fox and Falco both have the same FS completely. The devs were so lazy that they literally just have the game call Fox's Final Smash when you use Falco's. Bowser's Final Smash, Giga Bowser, is completely different from that. Not only is it a completely different character, instead of it being treated how you think it would in your mind, Giga Bowser is actually just another character in the game. He has his own character file and everything; if they wanted to they could have easily made him playable from the CSS. Same with Warioman.
But yeah, probably the reason why it's limited in Brawl is because of the way they coded it and maybe (just an assumption) framerate drops with tons of Final Smashes going on at once. They can definitely have more than one Final Smash at a time in a match in the next game... assuming they don't make them the same way they did in Brawl. And thus meters would certainly be possible.
HOWEVER, I really doubt Sakurai will go that way because he doesn't want to "copy", he wants to make things different. He wants to give the fighting game elements he adds Smash flavor, and the Smash Ball was a way to make it Smash flavored. Unfortunately, it was terribly executed IMO.