I don't know. I have little tennis knowledge.
But most people seem to agree with me.
It's MTF, this should be your first clue that something is amiss
Seriously though, the only reason Kei isn't in the top 10 already is his terrible luck with injuries. He's a brilliant player and shotmaker: his bagel collection includes Djokovic, Ferrer and Raonic (relevant because of how tough it is to bagel a guy with Raonic's serve), with breadsticks over Tsonga and Berdych. Not only can he hold his own against the current top 8, he actually 'dominates' against Ferrer, Tsonga and Berdych.
For me, the biggest worry with Kei is his health. If he can play a full season with no injury problems, there's no way he will not be at the O2 for the WTF imo. His main challenge (apart from staying healthy) would be to make his serve a weapon (as far as possible) to be able to compete with the very best players.
As some commentator said, if you can serve very well, you'll get to TB at minimum. Raonic won San Jose playing TBs in every match (won them all).
Yeah, San José. You obviously need more than just a huge serve to be among the elite. For instance when he faced Kei (a great returner) in the Tokyo final, as soon as Kei got a read on his serve he bagelled him. Can Raonic improve his game? Yes, he can, but Kei has more game and potential overall imo - much better returner (by a huge margin) and shotmaker, moves better, higher tennis IQ, Raonic's only edge is serve and that will only take him so far especially in this slow court era.
Even this year, despite the ranking Kei had the better year in the big events for sure, with QF at AO and OG. Raonic is currently ranked higher because he managed to play a full season while Kei's progress was constantly stunted by injuries (in crucial junctures too like after his AO run and after winning Tokyo).