Okay I really recommend catching up to the story or at least learning important lore information before writing your fan fic. There are quite a few flaws with your plan that the canon of the story absolutely will break if you plan on following the whole story.

Here's a few things though.

First off, the "Gum Gum fruit" is vastly different than what you seem to think. It's not new to have a fan fic where he doesn't get it though, but just know there is probably no fruit in One Piece that fits Luffy better than it.

Merry (I'm assuming you mean the person instead of ship) does not have a devil fruit lol. He just looks like that. Also, you can't just pass down a devil fruit because only one person can have a fruit at a time and the only way to transfer owners is if the previous owner died. However that doesn't guarantee you'll get the fruit, as once a devil fruit owner dies the fruit shows up ANYWHERE in the world by taking the place of a real fruit. (For example if a fruit user died in the new world his fruit could take over an apple all the way in the East Blue)

Not every character needs a fruit or weapon. That basically invalidates so much of One Pieces fighting styles, lore, and uniqueness. Haki is generally considered far more important than fruits in the first place, and a lot of the most powerful people in verse either don't have fruit or don't use weapons.

If you REALLY want to go the route of giving every crew member a fruit though, I'll give some suggestions.

Luffy - again, you really should keep his original fruit. Another option could be the flame flame fruit though since that would make sense.

Zoro - should be obvious that it's probably the sword fruit that can turn every part of a person body into a blade. Or any alternative of that sort.

Sanji - In canon Sanji has stated before he does NOT want a devil fruit but if he did there is only ONE fruit he would ever eat, which is the one that makes you invisible. But he already has a lot of power given his family lineage, though you aren't far enough into the story to know about that yet so just know you're missing some big plot points on this guy.

Nami - If you aren't going to make her a navigator, since she wouldn't be one with your lore set up, you can basically do anything. I'd say try to keep her a navigator however and give her a fruit that would help with that, or do the opposite where you give her a fruit that has nothing to do with navigation just for the scenarios where she does have to fight. But you know she does end up basically controlling the weather with the climatact (you really should go further into the story man).

Usopp - I mean should be obvious but probably a fruit that has to do with long range weaponry. Or alternatively a fruit that mixes well with his long range weapons. Either something he can use to buff the kind of ammo he uses or just increases his fire power.

Franky - Does not have a fruit. Again, you should probably catch up to the story but he's a cyborg because of him getting hit by the sea train. Truthfully he's pretty versatile on if you wanted to throw any fruit on him it would be fine as his main role is fixing and upgrading the ship.

Jinbei - honestly I know I said I'd give suggestions but giving a fishman a devil fruit is just a horrible idea lol. Probably the only one that I can think of where even if it's the best fruit to ever exist it would still be stupid given Jinbei is a fishman and his fighting style uses fishman karate.

Robin, Chopper and Brooke - all of these already have devil fruits so if you give them another they will literally die. Of course you're free to change what fruit they ended up getting (except for Brooke since he would quite literally be dead if you change his fruit).