I need suggestions on which nation is an interesting middle naval power to play (not USA, Japan, UK, Italy, France).
I'd hesitate to recommend Spain personally given how the SCW kinda dominates your entire pre-war phase where most other naval nations would look to modernize and expand leaving you to play catch-up.Holland, Spain, Turkey, Sweden, Canada and Brazil.
You can't refit your ships from Brazil though for some reason, seems like an oversight.
Canada ???Holland, Spain, Turkey, Sweden, Canada and Brazil.
Canada ???
Whilst Canada and Australia can certainly get some rather decent research bonuses they quite pale compared to a number of newer focus trees.The Paradox design team bestowed HoI4's Canada with powerful research focuses for cruisers and destroyers.
CAN has 2 focuses that give CAN 2.0 year ahead of time research bonuses x 100% (IIRC) for cruisers (light or regular).
CAN has 1 focus that give CAN 2.0 year ahead of time research bonuses x 100% (IIRC) for destroyers.
This allows CAN to have 1944 technology by 1939 or earlier.
Couple this with CAN superior military high command (particularly in the ASW realm) and a player can have fun ("fun" ? t'is subjective) sinking German U-boats.
The big-picture concern is that the designers don't take into consideration game balance. Each iteration of the game sees the designers infusing more and more capability into the game. This leads to the vestiges of earlier designs (e.g. SOV focus tree) paling in comparison. (This paragraph is at odds with another Paradox customer who stated that Paradox game designers at Paradox and their studios are too concerned about balance at the cost of fun.).
That being said, Canada has much naval potential in the vanilla version of HoI4.
Much.
And CAN is a fun ASW nation to play in multiplayer when GER goes all-in for a U-Boat strategy to attempt to strangle Mother England.
Canada to the rescue!