Thank you, first and foremost.
On the topic, I didn't think he would have, but after Hachinosu and this page:
I think Oda wants to make him as strong as he can during the actual plot of the manga and not in some epilogue EoS. Also, after having ruined and basically off-screened Smoker to oblivion, Koby is the only new-gen marine he can really use for this. Fuji is another "good" Marine like Koby, but it's not really new gen, even though he was new in our eyes.
About how, there are three options:
1) he'll fight Luffy sooner or later. He said back in W7 and repeated it very recently:
2) As much as some wouldn't like this, Luffy is among the other things the new Roger too and Koby is the new Garp... and Teach the new Xebec. With Oda, loving parallels and all, as all Japanese do, there is a very good probability we are going to have a God Valley 2.0 in which Luffy and Koby fight BB and his BBP as Roger and Garp did with Xebec and the Rocks.
Not to mention, this is not just for the sake of parallels:
Koby actually has a deep connection with the BB's plot already. Koby was kidnapped by him (kind of), he was rescued by Garp, his mentor, Garp went down trying to save him, he unlocked his "legacy" in Hachinsou, etc.
Koby is deeply connected to BB even now, even taking the GV's parallel away.
3) Koby, being a new-gen good marine and all, and the most important one given that pic I posted above, I think it's almost a guaranteed pick against the WG. I think there are high chances he's fighting a Gorosei (+ Shamrock) along with Zoro, Sanji and Sabo.
So, those 3, or some of those 3, even though at least 2 are sure for me, are all ways in which Oda can showcase Koby being prime Garp's level or even above prime Garp.