Silent_stiele
Alright so should our dear MC get a DF, now that's a hard question ! There are pros and cons to both. For now and as far as cannon goes Mythycal Zoans are the absolute best DF, they grant you an enormous boost to regeneration/stamina/raw strength while adding an extraordinary aspect to it depending of the type of creature, like Marco/Kaido, a perk Ancient Zoans don't have. We saw so far that Logias get less OP the further you go into the strength ranks. Except maybe Kizarus fruit and good ol' E=m.c2 will keep up with the big bosses. Moreover the 5 Elders all got Mythical Zoans for a reason, same as Luffy. If he can get one, a mythical Zoan, I think he should, but it should be a brutal one, one which goes in pair with his fighting style which is speed and offence. Sofar there is a pretty original Mythical that we haven't seen, well 2 that I can think of that could be awesome. 2 of the 4 Holy Beasts, Azure Dragon, Kaido got it, Phoenix, Marco got it, which leave us with the Black Tortoise (would love for it to be Vergo's mythical considering his pure strength and black haki fighting style) and my personal absolute favourite White Tiger. In mythology it was an absolute brutal beast, really bloodthirsty and emphasizing on sharpness and speed which is a perfect fit for our MC !
For the White Tiger DF you could get him to materialize Killing Intent or make it an extension of his Conqueror's Haki ! Now though he should be getting it really soon, since we saw that Mythical Zoans are nightmare to awaken with both Marco/Kaido still not able to when they got their fruits for decades. Plus if he gets it in his early teens he can grow with it. Plus I think it goes perfect with his personnality/style.
Considering his talent in observation haki, the perfect fit for him would be the Goro Goro no Mi, but i understand if you do not go with it, as it is a little overused. If you do not mind an original devil fruit, i would recommand the speed speed fruit, which would enable mc to be fast, even faster than kizaru once awakened, with a smart mc this fruit would be wicked strong, from speeding his metabolism to phasing throught matter by vibrations his own body fast enough, once older and with a good grasp on his fruit the mc would be near untouchable, his only weakness is that he would be a glass cannon if does not master armament haki, in term of fitting the verse considering the Strenght Strenght Fruit exist i see no problem with the Speed Speed Fruit existing