(Firstly, I'd be quite surprised if it DOES exist in any meaningful fashion, given that Paizo's business model seem to be much more AP-centred rather than rules/supplements centred and I think they'll probably keep it that way, and I ESPECIALLY don't think they'll be considering 2e until they've got D&D 5e in their hands to analyse, but that's not what you were asking...)
PF has many of the main issues as 3.5 did, inevitably, given its committment to back-compatibility. As such...
- fix the diverging maths at high level (PF is an improvement on 3e with its various fighter bravery bonuses etc, but something more structural is needed)
- trim down monster/NPC stat blocks (not necessarily to the one-page 4e size, but at least make them a bit more easily handleable!)
- reduce layering of combat buffs which require large-scale on the fly recalculation of stats, particularly at high level
- some sort of hero point system to give a level of narrative control - I thinik Paizo already like this approach given the number of their APs which hand out rerolls or bonuses through various Harrow deck mechanisms etc
- a simplified mook/minion system for unimportant flunkies - 4e had the right idea there
- it might be a sacred cow (longswords not doing 1d8 all the time! Gasp!), but rebalance the damage output of weapons so that the dice count for more and the bonuses less. Again, this is mostly a high-level issue, but it's a bit ridiculous when you're rolling 1d8+27 hp of damage. Have level/specialisation bonuses boost a weapon's dice size, or cap bonuses at the max damage of the base weapon, or something.