If I remember right when ep3 was here the servers were 'busy' and the lower maps were full. It seems the major changes instituted in ep4 were the changes in the dd1, dd2 portal system which allowed both factions access to the maps 100% of the time and also the elimination of the 7 level party restrictions. These 2 items or variations would seem to be an easy thing to bring back.
1. make dino/desert maps limited to level 60s and below so that people could grind there without 70s coming for easy kills. It would also require players to work together instead of soloing for level 6's and relegate 61-70s to the new maps.
1. make the Karis relics the key to opening the portals so only one faction has access at any time forcing high levels to pvp in Karis to gain access to the other embedded maps in desert/dino.
1. make dino/desert non-pvp like the auction house.
2. re-institute level restrictions/penalties with the 7 level max difference in levels. This would mean that veteran players alts would have to grind instead of plvl thereby increasing the number of players in lower maps and increasing retention of new players who could then find parties, make friends, etc.
People stay because they have an emotional/time investment in something. Going from 1-60 through plvl greatly decreases that 'investment'. Being always alone grinding also decreases the 'investment'.
I've played this game for a bit and it seems that a balance between paying and non-paying players is desirable. Aeria I think was a bit short-sighted in giving the paying players so much advantage over everyone else. I'm sure they did what they needed to for their business, but I think in some ways more volume of players potentiates the profits for Aeria in a way that we're not able to calculate. History might hold the answers for them in the future.