I disagree the most obvious hole in our lineup is catcher. On the current roster it's pretty clearly 3B, CF, and SP. As far as once you start inserting prospects you have to take diminishing returns into account, and then it becomes just CF and SP. There are 3 outfield positions, 5 starting pitching and only 1 starting catching position on a roster. Of the Jays top 15 prospects only 1(Martin) might be an outfielder. He also might be an infielder. If he is an outfielder we don't know if he can stick at Center. He only played 40 of his 140 college games as an outfielder and only 25 of those in center. I like our SP pool. But as much as I love each guy individually, the bust rate of SP prospects in enormous. You can't just assume current holes will be filled. Meanwhile the Jays have 5 catchers on the 40 man. 2 of which are top 10 prospects(Kirk, Moreno), another is an average MLB catcher right now with much bigger upside(Jansen). The other 2 are a decent prospect(Adams) and an ok backup with both extremely high end and extremely low end MLB results(McGuire).
It's much more likely that the 1 catching position will be filled successfully by 1 of those 5 guys(3 starting potential) than the 3 OF spots will be filled by the current 2 starting caliber outfielders and a great prospect who's a maybe outfielder, or that the 5 SP spots will be filled by Ryu and our 4 SP prospects in the top 10. Long term 3 guys to fill 3 spots(OF), 5 guys to fill 5 spots(SP), or 3 guys to fill 1 spot(C) I'm betting on being successful on the catching side of things as the mostly likely of the 3 here.
I think pitcher and outfielder, specifically centerfielder as the biggest 2 needs. 3rd is a current need but it should be short term given Groshans(SS/3B), Martin(3B/OF), Martinez(SS/3B) as on the way. Which is 1 of the reasons I'd have LaMahieu further down the list.
If we only can have 1 big name my list is pretty easily
Springer
Bauer
LaMahieu/Realmuto