I have a suggestion or two for making INT better, and then some more broad ideas.
For INT: I think Perception should have some of its responsibilities moved to Investigation. I think a decent number of tables were using Investigation as something more like 3.X's Search skill, and not the sort of metagamey "solve this puzzle for me" skill.
For STR: I would like to look at buffing STR a little bit, both defensively and offensively. As was mentioned earlier, Strength based AC caps at only 1 higher (magic items not withstanding). I would propose that sheilds offer greater benefits to those with higher strengths, probably a flat +1 AC if Strenght is at least 15, and another +1 if at least 19. Weaker holders are less able to brace and keep the sheild between them and the attack, and it gives a stronger niche for sword and board type play.
I also think 2-handed weapons should get something like 1.5 X STR mod added to their damage, rather than the normal 1X. Strength might be a bit of a specialization stat, offering either higher damage output, or higher AC than DEX, but not necessarily both.
Final observation: I think having each stat give a meaningfull buff at 11-13ish, then weaker buffs (if any) higher on is ideal. I would like scores to matter for more than just the modifier associated (so odd numbers have a reason to exist) with them, and this would be one way to do this. Being stpnger in the lower range, then having diminishing returns makes some investment more likely to happen, but doesn't needlessly reward good main stat synergies.