My initial idea was that the tournament is held in a giant cemetery where all the former players have been laid to rest.
[Rule 1]
Before the match starts, each coach adds D3+2 tombstones to his half of the pitch at least two squares away from either side-line, the end zone and the line of scrimmage. And at least one square away from each other. These tombstones then scatter three times like an inaccurate pass to determine their final layout.
Due to the uneven layout, coaches MUST swap ends at half time.
In most respects, the tombstones are treated as prone players - players cannot move through the square, the ball will automatically bounce if it enters the square.
However, if a player is blocked and pushed back into a tombstone (and this can only happen if there are no other free spaces, same as surfing) on either a pow, push or defender stumbles then treat the result as a POW. Additionally, add +1 to the armour AND injury roll to represent being slammed into a large, immovable chunk of stone.
If the player is removed from the match, the attacking player may follow up as normal. Otherwise, all players involved remain in their original square.
[Rule 2]
On a night like Halloween, in a sacred Blood Bowl cemetery, sometimes Nuffle's presence can be felt a little more strongly than usual. Any killed players (after any apothecary or regeneration rolls) may find that they are not allowed to rest just yet. On a roll of 4+, the player remains on the pitch, dead but still playing. The player suffers -1 AG and -1 MV but gains +1 STR. Player will also gain the Loner and Bonehead skills as the rest of his team are understandable disturbed by this turn of events, as is the player himself.
Finally, Nuffle occasionally rewards outstanding players with a second chance. At the end of the match, roll 1D6 for each (rookie?) resurrected player, if the roll is equal to or less than the SPP earned by that player in this match, then they fully recover from their unfortunate, premature death. All stats and skills return to normal.
Overall, I'm not wanted to upset the game balance too much. Rule 1 should lead to an increase in casualties and will probably make caging more awkward in places. Rule 2 will counter-balance the increased casualties to an extent. The rule regarding their full recover at the end of the game was really aimed at new teams, to help avoid losing SPP, which is a bigger issue for them than teams carried over from the previous league. I have no issue with some of those teams being trimmed.
I saw the Nuffleween tournament rules, and am considering redoing the weather and kick-off tables in a similar fashion to add some thematic flavour without changing the rules.
All thoughts and suggestions welcome, have at it!