Someone can correct me if I am wrong, but I believe the plan is to not go beyond Velious. I think there is one more patch in the works but no more expansions.
That being said, P99 contains levels 1-60 and won't go beyond that.
There are plenty of players on blue (non-PvP) server. You can easily find people to group with, but population is definitely skewed in favour of some specific zones. For example, at a low lvl, it is common to group and exp in areas such as G Fay, Crushbone, Oasis, EC, unrest, MM and so on. Whereas, zones with similar level creatures such as Qeynos Hills, Toxx, Runneye may be barren.
You can check the live server population numbers on the wiki. You'll see that the blue server often has more than 1,000 players in the evening. If you haven't used the wiki, check it out and get to know it. It's a great resource.
https://wiki.project1999.com/Main_Page