So we are getting a playtest in August, ex Pathfinder player here and reasonably enthusiastic.While I do not expect PF to clone 5E or any other game there are a few things I associate with 3.x games I think should be retained in Pathfinder while other games I think also have some ideas worth mining from.
If I have to guess Paizo seems to have a mostly complete edition ready for launch with a playtest to tweak it a bit perhaps. Its a very short test compared with 5E but they had to rebuild the game from the ground up after 4E. The following are thigns I would be looking at.
5E.
OKay okay okay I get it, 5E may not be to popular around here and I do not think Paizo should try and clone it. I play it because its the best D&D around atm and a lot of new players like it, its not perfect. I lean a bit more towards OSR stuff perhaps but 5E will do. 5E however had a lot of great ideas in it some recycled from TSR editions of the game, others new concepts or even from 4E. Paizo has already been influenced by 5E I think but there are some other htings they could look at or be worth discussing. For example 5E caps ability scores at 20, AD&D capped them at 25 while B/X capped them at 18. I don't think capped scores would have ruined 3.0 or 3.5 had they used them.
OSR Games
Over the last few years I have played some BECMI games (well more B/X). Some of you may know what B/X is but it is a older edition of D&D concurrent with 1E that was a lot more simple than AD&D. For example you could only play 7 classes and races (race= class unless human then you had 4 choices). Now I would not do that to a 3.x game but the way saves scaled at higher level, smaller numbers, and perhaps weapons and armor topping out at +3 could be worth a look. Oh OSR spells also tend to be weaker than 3.x spells (and 5E). SR/MR could be worth a look as well. Concepts not clones (THAC0 can stay in OSR/TSR land).
Star Wars Saga Edition
The other 3.75 game, had 4E been a bit closer to this instead of the 4E we got Pathfinder may not have existed. There is not a lot here I would want to use but there is the occasional thing like prestige classes, some talents, feats and skills and how they interact here that is fun.
4E
Yes strange is it may sound (I dislike 4E a lot) but it did do a few things OK. For example weapon properties, critical hits, some feats and class options (as in starting options/archetypes, powers lists not so much).
D&D Minitures
Back in 3.5 they had a minis skirmish game. I liked their system of SR/MR and how it worked with a single d20 roll and caster level did not effect it. Tweak to a high/medium/low number perhaps (6,11,16) and you need to beat that number on a d20 roll.
Mostly I would want fixed math and a few of those games can help here in that regard. Smooth out the math, reduce rocket tag, make martial a bit more resilient vs magic at higher levels.