Hello! You suggested I make an issue here so that's what I'll do. :)
I was wondering if it would be possible to add some way to differentiate between fishing rods in terms of the catching minigame and perhaps even fishing rewards.

Currently this is what fish stats look like:

"salmon": {
  "idle_time": 10,
  "top_speed": 1.5,
  "up_acceleration": 0.8,
  "down_acceleration": 0.8,
  "avg_distance": 55,
  "move_variation": 10
},

`

My suggestion would be to be able to modify these stats with a fishing rod similar to this:

"morefishingrods:energizing_rod {
  "idle_time": 0.9,
  "top_speed": 1.1,
  "up_acceleration": 1.1,
  "down_acceleration": 1.1,
  "avg_distance": 1.1,
  "move_variation": 0,
  "escape_speed_multiplier": 1,
  "fishing_bar_size_multiplier": 1,
  "treasure_chance_multiplier": 1,
  "experience_multiplier": 1.5
},

`
With this, the Energizing Rod from the mod More Fishing Rods (entirely fictional mod and item) would increase the top speed, the acceleration the average travelled distance of any fish by 10% while also decreasing their idle time by 10% to give the player a bonus of 50% more experience.

In the end, with the Energizing Rod, the Salmon would behave like this:

"salmon": {
  "idle_time": 9,
  "top_speed": 1.65,
  "up_acceleration": 0.88,
  "down_acceleration": 0.88,
  "avg_distance": 60.5,
  "move_variation": 10
},

`

I know as it stands right now, you can change the values overall but not taking into account what fishing rod the player is using. I think this would be a fantastic addition for modpacks or singular mods in general that add more fishing rod variety to allow the player to really feel the progression of their fishing journey and branch out into using different tools for different occasions.
Would it be possible to consider this for a future update?

This is a very creative and interesting suggestion! I'll mention this to the owner of the mod and I'll get back to you.

P.S. In your example, the rod's "move variation" multiplier is set to 0, but the final salmon result's "move variation" is 10, same as the original. Was this intentional?

This is a very creative and interesting suggestion! I'll mention this to the owner of the mod and I'll get back to you.

P.S. In your example, the rod's "move variation" multiplier is set to 0, but the final salmon result's "move variation" is 10, same as the original. Was this intentional?

Oh my bad! Should have clarified that a little. I wasn't 100% sure what move variation actually does so I left it at 0 even though it should have been 1.

Read the code and it's the variation of the targeted Y position of the fish in the minigame. That makes sense. Then it should have been 1 from the get-go for that rod and I accidentally left it at 0. :)

Also thank you! I tried to come up with a solution that wouldn't require a whole new system to be coded but still impacted the gameplay a lot in a customisable way. Excited for future updates!

Yeah, I'm currently working on making documentation for all of Stardew Fishing's features, including the technical stuff. And your suggestion will be included in v2.4!

Mod owner here,
This is a fantastic suggestion! Would be very useful modpack makers too. We'll see if we can implement this!

Your suggestion has been implemented as of 3.0 which I just uploaded! I called the system, "minigame modifiers", and you can see an example of it in action here: https://github.com/BrownBear85/stardewfishing/blob/master/src/main/resources/data/stardew_fishing/minigame_modifiers.json