(8 players) Move the gold farther away
My group plays Saboteur most frequently with 8 players, and we've developed some house rules for that size that are probably not applicable for smaller groups.
We were already playing with a guaranteed 3 Saboteurs in an 8-player game and with the Saboteurs having knowledge of each other, but wins for the Saboteurs were still very rare and highly dependent on an ideal draw for the Saboteurs' hands.
We recently started moving the gold in this case - the middle treasure card moves back, so it is now 8 cards from the starting point. The two side treasure cards move out, so that they are 7 cards away in the "forward progress" direction, but 2 cards away from the middle.
We unanimously agree so far that this has led to some extremely fun games - the wider board gives more space to improvise, but punishes good dwarves who dig all the way to the wrong treasure card. We had one win where the Saboteurs didn't even play any "dead end" pieces, but used their near-monopoly on Map cards to confuse the good dwarves and get them to dig to the wrong treasure. At that point the good dwarves couldn't QUITE make it back to the correct one with the pieces available, but it would have been a trivial correction in a standard game.
Spreading the board out like this also makes the pieces that make "sideways progress", especially the one that splits three ways (), much more useful. Under the normal rules I will almost always consider a straight-sideways piece to be useless as a good dwarf (though obviously it occasionally comes in handy), but with the spread board these pieces can be very key.