GnomeBob
Let HK eat a zoan!!! (i advice hito-hito model:whateveryouwant or you can take the mascot route) As for the lightsaber the zoan doesn't fit and in canon just zoan can be feed to inanimate object. If you want a lightsaber upgrade you can let vegapunk teach him(or find vegapunk old lab like franky), is base in tech should not be bad considering that he can thinker with is lightsaber and well starwars tech knowledge are more advanced, but they are not as wacky as one piece (i mean franky literally use cola as fuel) or you can go with the "wano" route, make a vibroblade with kyber crystal the Je'daii didn't use lightsaber, you can make a black Force-imbued blade with haki. (google tell me that's what the Je'daii used, from the image there are rune engraving on the blade you might bs your way thought with divine script and give nen ability to the blade, if you want) You are doing amazing in not butchering canon, if you give is lightsaber a logia/paramecia you need to give logia/paramecia to the item of other player as well... I think is fine to put a non-canon devilfruit, but i dont think is a good idea breaking canon "rules" for a cheap upgrade when there are other way, is also more in line with the mc, with is personality i dont think you can have him kill somebody for the sake of stealing a devilfruit.
Buggies Chop chop fruit allowing him to separate his blade into pieces expanding into a larger blade chain blade or letting him make air cuts that launch his lightsaber sections could be neat if he isn't up for killing nico robin for her hana hana no mi. Wopols Baku baku no mi could make for a fun way to upgrade his lightsaber with special materials he finds.