At the same time though, I can't help but feel like it's underwhelming. It's not a bad idea, it's certainly abstract, but is it absolute endgame quality? That part I'm not so sure about.

Sigh...I didn't want to move the thread in this direction again, but I suppose I ought to say what I imagine a number of people are thinking already: The sphere idea feels a lot like the Ascended Deedle Ball idea with all the humor surgically removed. What's more, my own level has sort of a running theme of spheroids building up to the joke battle with you-know-who, and following this up with a battle with a much less, ah, shall we say, "distinctive" sphere is just going to seem awkward, and very likely to leave the player thinking "Was this originally intended to be...something else..?). If nothing else, we should strive to avoid any sense of disagreement over such questions of overall vision coming through in the hack itself.

That all said, I found Ludus's Blueberry Lord idea compelling not merely because of the "ha-ha, blueberry" apsect (though that's certainly a contributing factor), but rather because it isn't actually a villain, but rather a character initially non-hostile toward Mario, who later becomes offended. This, I think, would be an decidedly more interesting approach to take with our boss here.

Taking this all into account, I would like to propose the following synthesis of some of the more popular ideas here:

My own stage, as mentioned before, ends with a joke battle with the normal Deedle-Ball. This would be our "fake" extra boss, and a shout-out to those who enjoyed the running gag. When this harrowing battle is ended, Mario will be sneezed skyward as described before (for even if we leave out the transfiguration part, I dare say this is essential), but instead of depositing him in a black void, it would instead deposit him in a sacred realm, where he meets his Spirit Guide in a false bonus ending. Said Spirit Guide could conceivably be realized in a number of forms, of course, but I think most of us can agree that it should probably be blueberry based.

The Spirit Guide will receive Mario warmly, and congratulate him on completing the trials and tribulations he has endured on his journey through these land of dreams. As a reward, he begins to reveal some great truth to Mario to aid him on his path to enlightenment, when suddenly Mario, or that fool Yoshi, or someone else related to our hero does/is revealed to have previously done something to deeply offend the Spirit Guide (ideally this would center around some minor and seemingly innocuous action performed at a much earlier stage in the game, its significance now being blown far out of proportion), whereupon a conflict breaks out, resulting in the true extra boss battle. And if you like, the battle itself could even involve the elemental crystal idea which seems to have gained some traction here--or better yet, elemental berries of the non-blue color, orbiting about their fantastical sapphire lord, like a host of ordinary serpents and lizards swarming about a majestic legendary dragon.

This approach has several advantages--it has a little something to please all the factions, but combines them together constructively instead of paring them down, it has a touch of a self-reflective parodying aspect while at the same time providing a suitably dramatic (by Mario standards) ending to the bonus world, and it fits a bit more organically in the context of the bonus world itself instead of having a foe just appear out of nowhere and justifying things with "Dream world, anything goes!". And due to the inherent potential for disjointedness collaborative works such as this have, such connections are important, adding as they do to an overall sense of coherency--and that's an impression we especially want to deliver for the absolute final end of the hack, no?

It was the Mario's Spirit Guide I was mainly referring about as the final boss. The deedleball with the nose and all, since it is not the final bonus boss, I don't really care with it, though it is a good idea, it's the FINAL boss I am most worried about being mistaken as something silly... when any final boss IMO should not be taken silly, even if it is a bonus. Look at Culex in SMRPG, bad-ass, yet has nothing to do with the story.

Even VIP 5 has a bonus boss.

But some we need a new bonus boss that looks different than the other bonus bosses.

Pray tell, how exactly are we going to introduce a blueberry in a way players will appreciate?