The idea of limiting player input into their character's actions based on INT is something I would not do. Whether or not your character comes up with brilliant or stupid ideas is completely driven by the player. It is part of the game. To play it otherwise turns the character into an automaton.
There are already some real benefits to high INT that have been listed already. This is an open ended game people. If the problem is a lack of mechanical benefits, then make some new ones.
There does seem to be a real shortage of spells beyond illusions that require INT saves. Make some new spells. Give NPC casters these new spells. Mix liberally with existing illusionist spells (which can be downright vicious against a low INT character).
Create some magic items that demand a high INT to use...I'm thinking really powerful intelligent weapons that will completely dominate a low INT character. That will make that Paladin wish he was smarter.
Create situations where high INT can have a real impact on the game - base them on the INT skills:
Arcana
History
Investigation
Nature
Religion
For example, The party finds themselves trapped in a room with magical artifacts that have nasty effects on low INT characters. A series of arcana checks are required on each artifact to disable them.
Etc.
It just requires a little imagination. Why do so many people act like if something isn't in the PHB then it's a problem?
There are already some real benefits to high INT that have been listed already. This is an open ended game people. If the problem is a lack of mechanical benefits, then make some new ones.
There does seem to be a real shortage of spells beyond illusions that require INT saves. Make some new spells. Give NPC casters these new spells. Mix liberally with existing illusionist spells (which can be downright vicious against a low INT character).
Create some magic items that demand a high INT to use...I'm thinking really powerful intelligent weapons that will completely dominate a low INT character. That will make that Paladin wish he was smarter.
Create situations where high INT can have a real impact on the game - base them on the INT skills:
Arcana
History
Investigation
Nature
Religion
For example, The party finds themselves trapped in a room with magical artifacts that have nasty effects on low INT characters. A series of arcana checks are required on each artifact to disable them.
Etc.
It just requires a little imagination. Why do so many people act like if something isn't in the PHB then it's a problem?