Here is a neat concept for you... World starts at 20% for the first month and then every week the percentage goes up 10% until eventually 100% of the players are attackable. That is 3 months until all are attackable. A decent amount of time for all to grow or for those that just want to remain small and harvest gold.
Certainly the developers could institute applicable coding for this.
Something has to happen because at 3k right now, I can't touch anyone on my island or in fact, anyone on islands close to me. At the very least you need to be able to attack larger players. I get that in a casual world you don't want any noobs or unskilled mowed down until they get a chance to learn but at least allow for the attacking of larger players.
Maybe limit it to those with the same number of cities. If you have one you can only attack players with one city; if you have 2 then you can only attack those with 2 or more. After you hit 10 cities there is no upper limit and you can hit players with as low as 5 cities. At this point anything is better than this 20% garbage.
As to the lack of players, to me it goes back to a couple things:
1. I got into Grep based on their tv commercials and loved it. I have not seen a Grep commercial in years, but see their other stuff all the time. It would help to have them actively recruit players again.
2. Gold use. As has been stated in other posts, when it was just for cutting time of building or recruiting it was good. As soon as they opened it up to what it is now is when players started leaving. In essence, you could buy the win in the one's leaving mind. I've been around since Kappa, US 11 I believe. Since Kappa I have had an odd ball Gold status because the Gold use was for cutting in half and the gold amount was very much random. If we went back to that kind of gold use it would put more pressure on strategery and less on the wallet.
Those are my thoughts, have fun ripping them.