For reference, this means pre-Reversion usable abilities would be Magic Guard, Illusion, Contrary (for switching into Sticky Web), Competitive, Defiant, Drizzle, Sand Stream, Drought, Snow Warning, Delta Stream, Primordial Sea, Desolate Land, Cloud Nine, Air Lock, Forewarn, Frisk, Intimidate, Levitate, Simple (albeit negatively for switching into Sticky Web), Slow Start, Trace, Unnerve, Aftermath (if you're somehow able to lose all your HP before reverting), Immunity, Imposter, Anticipation, Aura Break, Clear Body, White Smoke, Download, and Synchronize.
Mind, not all of these are useful. Some of them are actually silly (Primordial Sea Red Orb Groudon?). And most of them are only very situationally useful. But... well, hey, why not have an ability that at least does something before you lose it?
To break it down...
I think Magic Guard, Download, Intimidate, Frisk, Forewarn, and Trace are the most useful.
Immunity, Competitive, Defiant, Contrary, Levitate, Synchronize, Clear Body, White Smoke, and Imposter are all situationally useful. Most of these depend on Toxic Spikes or Sticky Web to be present to do anything. Or to switch into Intimidate.
The rest are silly or outclassed, but are listed for completeness' sake.
There might be a few others that could potentially be useful if using Baton Pass. Maybe.