Assuming we remove Head Smash, Regenerator would be better than Magic Guard in most circumstances. With Magic Guard, the damage Cruci does manage to acrue at least sticks around, but with Regenerator combined with Cruci's speed and bulk, not even direct damage would stick around for long even with factoring in hazard damage.I also support Deck Knight's proposal of changing Mega Crucibelle's ability from Magic Guard to Regenerator. It will become a lot riskier for Mega Crucibelle to spam its strongest move and easier to securely revenge kill when it's no longer outright immune to residual damage. It also enables Mega Crucibelle to retain many of its unique traits, overall identity, and a pretty good level of viability while opening up more avenues for reliable counter-play.
Now this is what I really wanted to talk about. I think this is a very interesting idea that I haven't seen mentioned before now. I'm not really pro or anti volt switch right now, but I'd love to see some discussion and have a couple things about the idea I'd like to mention. When it comes to damage, volt switch may be able to do more than U Turn in some matchups, but it doesn't do more damage to anything than what Stone Edge, Head Smash, or Gunk Shot would do. I think what The Idealistic mentioned about Volt Switch requiring the user to think rather than mindlessly pivot is the main draw of this idea. If the opponent has an electric immunity on their team, the Crucibelle player has to risk losing a lot of momentum by either Volt Switching into a Ground type or by hard switching into the wrong Pokemon.Another potential nerf I considered was replacing U-Turn with Volt Switch. Coming off its notably worse Special Attack stat (especially when a stat dropping nature and lack of investment is accounted for), it can't freely chip checks/counters with its pivoting tool to the same efficiency that it once could. Even better, it can no longer mindlessly pivot in and out of battle when the opponent has a Ground-type, lest it risk sinking a significant amount of momentum. On the other hand, it does a fair bit more damage to a few checks (such as Argonaught, Celesteela, Kitsunoh, Mawile, Mega Aggron, and Revenankh) and alleviates it from making contact with users of Rocky Helmet and/or Rough Skin/Iron Barbs. However, the downsides of Volt Switch over U-Turn at least make this avenue worth considering.