One limit is that if you've searched every square of a maze, you're guaranteed to have found the exit. So if you want to have a harder maze than that, you must

One limit is that if you've searched every square of a maze, you're guaranteed to have found the exit. So if you want to have a harder maze than that, you must visit each cell more than once. i.e. lots of backtracking. If you have N dead ends (i.e. leaf nodes), the node before them must be visited twice (down and back up again). The previous node can be visited at most 4 times (in, down and back for each edge, and back out again). Past that, limitations due to geometry will slow down the growth of possibilities.