How do you propose this ground pound will operate? I've seen a lot of people suggest move changes for characters because it will make them more similar to the characters' games, but they can't effectively justify how it would be a good idea from a gameplay perspective. A good exercise for this kind of thing is to think of how you would design the character if they were completely original to Smash, never having appeared in any other games at all.
Of course one looks to the characters' games for inspiration for moves, but this is a fighting game; the priority should be how well the character plays in Smash, not their consistency with their own universe. Lucas and Ness are the first examples that come to my mind of characters that take necessary liberties with their universe; they still use PSI like in their own games, but many of the PSI techniques they use in Smash were only useable by other party members in their own games, since staying truer to their games in that respect wouldn't have worked properly.
Please say in as much detail as you can:
• What properties this new move will have
• How it will benefit Luigi
• How it will change his gameplan/playstyle and feel
And correspondingly, how losing his old dair will affect those things
Both Mario and Luigi already have useful meteor smashes (their respective forward air and down air). As I understand it the ground pound would remove a versatile and useful move (his existing down air) without enough benefit in return. It is a very important finisher, without it the only move he can use from the air that knocks the opponent far away horizontally is his forward air; neutral air launches vertically while up air and back air are both too weak and more suited for combos and edgeguards (in my opinion and experience; you are welcome to disagree). I also have concerns about the combination of a move that forces Luigi downwards and his poor aerial mobility.
There is already a thread about Luigi's meteor smash, so you would be better off taking your suggestion there; this board isn't exactly well populated so we should congregate in a small number of threads to maximise useful attention
(http://smashboards.com/threads/how-op-would-luigi-be-if-he-had-a-reliable-meteor-smash.341733/).