A tileset is a large undertaking and it's difficult to maintain a consistent, refined style. If multiple artists are working on this over time on a hobbyist basis, the end result won't be too great.
In this case, it would be far better to commission a new tileset by a professional with a clear and comprehensive brief.
A professional will deliver a good tileset within a deadline and in a timely fashion.
The only other option is for a volunteer to work on this in a professional manner, with a spec, milestones and deliverables. In this case, an agile, iterative project management model would work best.
In either case, you need a project plan before anything can be expected to happen. This is the type of project that is doomed to take literally forever otherwise.
Perhaps Zone in general would benefit with this kind of approach to deliver some large scale changes that everyone here has rightfully identified. Without, you'll only get short term projects that are a rush to complete I.e seasonal releases.