If you're talking about building new things on an existing line, I do kind of think you should leave it be since the trail has been established there for a long time. If you're talking about building new off-shoots with cool stuff that connects back to the existing trail, build away. No one has a right to ***** at you if they aren't out there maintaining the trail too. And if they are out there maintaining, work together and add to the trail in ways that work for everyone. As long as you're not messing up their current stuff, what's the big deal? We've got a couple of big gap jumps at my local trail. Unfortunately there aren't any jumps close to the size to work up to hitting the big ones. It sucks, and I wish I could go out there and put some dirt in the middle so that I could safely practice jumping them, but I'm not the one that takes care of that trail, so too bad for me. If you're taking care of it, add whatever you want.