Ash's rivalry with Paul had a climax centered around Brandon, an established, well-known, high-ranking trainer who Ash had defeated in the past. It was at that point in the rivalry that Paul started to look at Ash as a considerable threat, rather than a brainless novice.
If the writers plan on making Ash's rivalry with Shooti as close as possible to his rivalry with Paul, they're going to need to bring back an old character to serve this role: A high-ranking, well-known, powerful trainer who either has been defeated by Ash or at least is knowledgeable of any of Ash's significant accomplishments. Again, the goal is convincing Shooti to view Ash as a considerable powerhouse, rather than a pushover trainer. What makes this situation complicated for the writers is that, unlike Sinnoh, Isshu is established to be far away and completely isolated from any past region Ash has traveled through.
The most likely trainer to serve this role is Cynthia. Not only does she make an appearance in the B/W games, regional champions tend to be well-known even outside of their home region. Shooti will likely recognize her as a powerful trainer (or, if he chooses to continue to be ignorant, he can find out the hard way by getting pulverized by her Garchomp). Though several plot devices, she can explain to Shooti (and possibly Iris/Dento) about her observations regarding Ash and Paul or Ash's league endeavors (Darkrai and Latios anyone?)
If the writers choose not to take the Cynthia route outlined above, then Paul will likely return in Best Wishes to fill this role. The writers could easily come up with a random treasure for Brandon to find in Isshu, with Paul heading to the region to pursue him there.
Paul could do some training at some of the battle clubs, gaining a strong reputation in that system, which would entice Shooti to challenge him to a battle. Paul defeats/destroys/demolishes Shooti, which leads to Shooti being interested in Paul's training secrets or whatnot. Somewhere in that conversation, Paul mentions his loss to Ash in the Sinnoh league, and then Ash's training style. This will also acccomplish the goal of Shooti beggining to take Ash much more seriously.