This conversation/comparison has been running through many Twins fan's minds for a while, now, and it was necessary to finally address the thought out in the open.
I see Wallner as more of a Kepler-at-his-best (2019) kind of player than a recent Gallo or, sadly, Sano type player.
Wallner has displayed great talent and a high upside at every level, but its going to be more of a mental battle than anything for him, base on his history. When he has stayed focused and worked hard through tough patches, he comes out explosively, like a rubber band releasing.
I think the key to him becoming a very good to great major leaguer is tenacity and Minnesota stick-to-it-tivness. And he is a Minnesotan.
Wallner has started each season slow, worked hard to figure out the fixes and then mashed. If his management, teammates, and fan base can have patience and keep faithful, history says he will come around and produce at a very high/successful/exciting level.
Gallo (often) and Sano (mostly) were strong starters who typically slowed down or were worn down by long seasons and/or injury and were not as mentally tenacious as Wallner has displayed.
My two cents.
Unfortunately, slow starters don't make all star teams, so I think he will be largely missed by the rest of the country's baseball fans, like so many other solid Twins players in the past, but will serve our team well.
A second season of great success due to the Twins youth movement (Julien, Lewis, Ryan, et al) may help garner some attention, but it will take post season heroics to make Wallner a household name.