[RPG] For a homebrew class, how can I offset the disadvantage created by not granting proficiency in one of the “big three” / “strong” saving throws

Spellcasting is not the only reason for a ranger to care about WIS.

Is spellcasting the only (mechanical) reason why a Ranger would care about having a high WIS

Wisdom is a Thematic Match for Hunters. Rangers are depicted and designed as hunters of various sorts. Perception and insight are wisdom based and are skills relied upon for spotting and intuiting the actions of quarry.

Potential mechanical side effects of favoring INT over WIS for a ranger

are there any other unintended side effects of making WIS less important for a Ranger?

• Favored Enemy ability: You have advantage on Wisdom (Survival) checks to track your favored enemies
• Foe Slayer: add your Wisdom modifier to the Attack roll

Interestingly the saving throw proficiency is not affected by favoring INT over WIS.

Other Considerations

Spell School Restriction

Swapping arcane for ranger spells on a horizon walker ranger is an interesting take, but seems like it is stepping on the toes of both the eldritch knight and arcane trickster. Both of those classes have the spells they can learn restricted to two schools.

Arcane Versatility

Neither arcane trickster nor eldritch knight can select any spell from their potential list at casting time. Both must select a limited set of spells when they level. Does the modification you're proposing include limiting the spells that can be selected from at casting time in a similar way?

Wizards and sorcerers are more versatile in the selection of spells that can be chosen at casting time, but these classes do not have the martial prowess of the ranger. Allowing an expanded list to be selected from at casting time might be overstepping relative balance with these classes as well.

There are several options:

• Heighten Spell Metamagic
  • Requires three levels of sorcerer, lets you impose disadvantage on any spell's save. Costs three sorcery points.
• Bane spell
  • Requires a level of Bard or Cleric, or Magic Initiate, or something else giving you it or a generic level 1 spell off the Bard or Cleric lists. Doesn't actually impose disadvantage, but rather subtracts 1d4 from attack rolls and saving throws for up to a minute. Requires Concentration. Affects multiple targets. At higher levels affects even more targets.
• Unsettling Words
  • Only available to College of Eloquence bards; again, not disadvantage, but subtracts a Bardic Inspiration die from the target's next saving throw.
• Bestow Curse spell
  • Gives a target disadvantage on any one save for a duration that varies with the slot level used. Requires 5 levels in Bard, Cleric, Wizard or a thing that gives you a third level spell off said lists or otherwise access to Bestow Curse.
• Any of the diseases listed in Contagion
  • While contagion is a 5th level spell, it specifically notes that the diseases it can curse targets with are natural diseases that exist in the world. The diseases can presumably, then, be contracted other ways and part of the effects thereof is having disadvantage on a save (there are 6 listed diseases, one for each ability score).
• Gold Dragon Weakening Gas
  • You'd need to bottle it or something, but this gas gives disadvantage on strength saves for up to a minute as one of its effects.
• Treasure from a Mummy Lord's Lair
  • Anyone who takes this has disadvantage on all saves till the curse is broken. If you carry some around, you can trick people into activating the curse, but you yourself will also be cursed.

There are many more options-- almost every casting class and subclass gets something that lets them force disadvantage on arbitrary saves-- but most of the other ones that are class features require being a higher level. There are also lots more options involving monsters than the two I mention, but most of them involve a lot of work to put together without lugging the monster itself around with you. There are also some magic items, of course.