I'm glad this thread was started. I agree with the amplified 'vs Environment' element here, however there are a lot of things that have worked and not worked in the past. I'll try to be as succinct as possible:
I don't think we can have a serious conversation about lengthening PVE map duration unless we adjust and accommodate the needs of both new and old players throughout the course of time on a map. There was a vote on map-size in which an even larger map size was the most popular, however I think this might be a misstep when it comes to actual retention. I believe it is possible to fit all the amazing biomes with a meaningful size and distribution into a moderately sized map - it simply takes the generation of many maps to find the right one. Towns can have just as much room as they ever needed, just as they did in the past. What is happening now on PVE is that there is no time to go and see others on the map. There is far too much walking. Want to build a rail to that other town? Now you have to spend 3-4x the time and materials to build that rail, not to mention the time it takes to ride on that long ass rail. My honest opinion is that we're not without buttloads of materials and building space even with a 5K map. Further, we're all growing older, we encourage a mature player base, and as students and workers, we're all wanting for this time that we deny ourselves by increasing the map size.
Speaking of materials... Underground has been awesome for a long time on P. I feel like we've gotten a correct amount of plumping. However trading has changed the world of PVE in a huge way. Diamond tools are not expensive and will probably never be expensive again. However there are rare materials seeded throughout the world and rare materials that most players cannot even get. I'm not sure how we've handled trades in the past, but I think we could rehash an old concept of trade signs and have a PVE "rule of 10". Simply put: lets come up with 10 simple, but expensive trades so people can get some bling bling. Almost no one wants a diamond block house, however someone might want a few sponges for a special build or lets just say... 32 diamonds equals a wither skull or something. And who doesn't want the occasional villain who releases a wither on your town. Everyday mining, grinding, building gets old, lets get some surprises from time to time - I haven't even seen a wither on this server yet. Typically the players could make the trade sign like in days past: [Trade] tradepostid
The tradepostid would be a pre-determined, publicly posted trade identifier that you could easily see by typing /trades
Then use the item in your hand, right click the sign, to get the trade. No backsies. The idea here is to make certain things available at all times, not just for the ones who find all the dungeons and special things first.
Feel free to flame this post at any time.
Moving on, I think overland could be more interesting. There have been times that we have gone way overboard... One has to be careful with the psyche of the average player on PVE. Lots of terraforming CAN be fun, but it has to be with limitations. I'm still a huge proponent of rivers that are a little bit larger. 20-30 wide rivers as opposed to 10 wide. This allows for some extra challenges such as large bridge building and coastal creations. We could also use good mountains - not overly massive or steep, just good mountains. It would also be kinda cool to seed a little extra diamond/gold into something like that too, so maybe you'd be creating a whole society of mountain miners.
Just some thoughts - maybe I'm nostalgic, but I remember some specifically fun things about PVE that could easily be put back into play.