There are a number of things that 2.0 could take from 1.0, monsters as Jynx mentioned was a huge boon to the living world feeling for me. Another which likely won't happen, but was more present in 3.0, is traveling - I love the feeling of scale that can bring and I don't exclusively mean "wait 45 minutes on a boat to wait 20 minutes for an airship to wait 30 minutes for an altep summon". WoW has shown you can keep things under wraps and still have traveling (the open world map also helped with that aspect in conjunction). I really like the idea of that feeling that begins when you start to travel, especially when you can tie it into the world lore (why are there no caravans between the three cities? Seriously no one trades? Train tracks in thanalan and no train?). Again 3.0 was a bit better, bigger zones few aetheryte and requires you to explore first - appreciate this, still missing a societal method of transportation though.. And a dream we get an overworld map, can make flying work in 1.0 zones without editing them by lifting off into the overworld map and only landing at designated locations, use a No Man's Sky sort of effect as a loading screen :P.
So I'm excited to see 4.0 just because they might keep doing large/exploration themed zones (not opposed to small zones, but not every zone (imo) needs an aetheryte). While sky is nice it will be neat to see what they will be willing to do on the ground as well (not a complaint of 3.0 just an excitement to see more).
The higher quality models (flower pot that had the same amount of polygons as a character

, huge problem for lower end machines.. I know..

) and better textures is missed (not always true, largely talking about the mipmapping resolution). With on the fly animation modification (does take system resources, especially if everyone has it applied) you could avoid forcing momentum with 1.0's awesome run animations - loved how it looked like you were actually running on the ground and not floating upon it (as an aside, get some stair animations.. lol). Would love to see a enthusiast setting~ I also miss some of the colors (2.0 allowed some very "modern" color palette that not quite WoW went away from some of the more earthy tones).
Obviously the cutscenes were very well directed.. felt like movie at some points. Though (as SE said) it is very costly and slow to get through the pipeline. They've shown glimmers here and there, hildibrand cutscene, major story moments, just hope to see them do it whenever they can find the space (imo the first time I saw a true form Ascian was amazing, 2.0 Ascians feel very organization 13 "meh, dem the bad guys" - glad they brought the model back for 3.0 though).
The amount of activity in 1.0 may have not been more than 2.0/3.0, clearly as you say way more NPCs doing things and talking, but there was an amount of character that imo 2.0 didn't take on with it. A lot of little things that made a whole (whole-ish? lol) experience, extremely high quality animations that grounded you in a large scale environment, which looked great on a macro scale (needed some work micro, like removing that Thanalan ramp and tri-hollow-tipped mountain that was 100% exactly the same as the other 5 you just went by). Monsters to be learned and feared, monsters to love being in the presence of.
Wonderful monster quirks like parasitic snurbles helping you out, goobbues eating the nearby vegetation, territorial crabs, and afraid critters. That monsters had sight, sound, smell, and true sight and you had to respect them otherwise you might be in a world of hurt (3.0 made some improvements on 2.0 in terms of monster strength, I feel like some monsters still have aggro types but you hardly notice or care because "lol 20 monster train"). Areas of the game that clearly told you you were not prepared "hi I'm level 80 tree, this is ground - you like ground". Beyond adding back the neat AI I imagine they could simply add a variable of monster levels, even "NM" class monsters. So you might want to check their guage before face pulling with your chocobo or you may want to check your gauge to have a very rewarding monster kill (perhaps they can use these to make light party monster roaming non-FATE exping viable (not OP, just viable)).
It may be too much FFXI but I find party interaction a bit interesting, maybe not exactly the skill chain system, but at least from the cutscene you see a Thaumaturge enfire an archer. Or a non-skill chain based ability that is more interactive (enfire is just a neat buff) would be like Guild Wars 2 where you had a firewall that lit passing archer arrows on fire (not opposed to a interparty member class system personally, but I can see it adding a very deep and high skill requirement if keeping everything else the same). Also miss being able to target monster's body parts to affect their in combat AI. So you had this quirky out of combat AI and then in combat you had another set, like not ramming when you break their horn and dropping their horn as a loot drop, or simpler ones like trying to get help (via running to or calling - dungeons do this already, but to my knowledge no world monster cares that his buddy is 1 malm away and just dies instead of trying to live).
While I certainly feel the mainstream environment won't take getting beat up by a rabbit anymore, I do feel like we can keep mining 1.0 for little gems. I'd probably like to see monster AI first, but a world feeling via outside of town network would have to follow. Give me that train.. xD. Also, Doom Train?