"A place is never hunted out", this has been proven over and over. Some places require more patience and improved hunting techniques, swing low and slow. Test the area, plant a coin or two and see what your machine says about them and readjust your settings as required. What are the best settings for Depth with your machine? Likely older coins will be deeper in the ground. Headphones really help to hear those whisper signals! Maybe you need to change your coil type or size to facilitate detecting in this old well detected area.
100 Years of being a park - lets see that would have started in about 1913! Back in those days people had bands out in the parks for people to listen to and or dance to, festivals were held there, amusement rides could have been setup there, foot races, football games, baseball games, Easter egg hunts (don't expect the eggs to still be good). I truly doubt it is hunted out I think a change in detecting methods will be of some use in your quest for "Old Lost Stuff" in that local.
I wish you much success........................63bkpkr
What does your research tell you about the park? Where did they setup the entertainment? What was the park Used For? Old newspapers might have some interesting inputs?