Tl;dr - See statements in bold, and possibly the last section
There isn’t necessarily a precise analogue between paper MtG mechanics and enable/disable effects here, but these seem to be the 3 primary classes of cases for comparison based on the current cardpool (i.e. excluding anything new we might see in Dominaria), although there are a lot of other niche scenarios where disabling has been used on individual cards outside of these classes (e.g. as a surrogate for “counter the first spell each turn” on Glyph Keeper):
(A) Disabling creatures can mirror “can’t attack or block” effects; in this case, there is no clear precedent for how Hixus should interact that I’m familiar with, as paper has no (frequently printed?) analogue of enabling where “until end of turn/beginning of next turn” instant and sorcery effects are concerned (e.g. with Alchemist’s Vial), and enabling in the case of permanents like Cast Out would generally be accomplished by removing the permanent or the permanent’s abilities, not by directly contravening the permanent’s “can’t attack or block” restriction.
(B) Disabling creatures can mirror “tap target creature” effects; in this case, Hixus should logically enable creatures disabled by Cast Out, because the analogue of Hixus’ enable clause would be “untap all tapped creatures your opponents control”, which unless otherwise specified doesn’t care about how those creatures originally became tapped or whether those creatures would normally untap during their controllers’ untap phases.
(C) Disabling creatures can mirror “exile target permanent until [cardname] leaves the battlefield” effects (with “[cardname]” being Hixus or Cast Out here); in this case, the precedent is ambiguous, because MtGPQ seems to have specifically avoided reproducing temporary exile effects verbatim other than Siren’s Ruse (which provides little guidance here), and Hixus’ enable clause could reasonably be interpreted in (at least) 3 different ways:
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“Return all creatures in the exile zone that your opponents own to the battlefield under their owners’ control”, which should enable creatures disabled by Cast Out (among other things), although paper MtG doesn’t have any effects exactly like this as far as I’m aware
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“Return all creatures exiled by Hixus to the battlefield under their owners’ control”, which shouldn’t enable creatures disabled by Cast Out (and incidentally should enable creatures that were stolen by Hixus’ controller and remain disabled, although that’s a separate issue); this is the closest template to what paper MtG typically uses for these types of effects, but it would differ from Hixus’ implementation both before and after v2.7
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“Return all creatures exiled by Hixus that your opponents own to the battlefield under their owners’ control”, which is closest to what Hixus’ implementation will be after v2.7 and represents a middle ground between 1 and 2
Overall, it’s difficult to tell what should happen based on paper MtG, largely due to the inconsistencies in MtGPQ; if given the choice, these are the adjustments I’d propose, depending on what the devs wanted to establish as the intended functionality:
1. Template Hixus and Cast Out consistently, no matter what
Both of these cards are derived from paper cards that exile a permanent until they leave play, and in this game both of them disable creatures until they’re destroyed; there’s absolutely no reason why Hixus should read “when this is destroyed, enable all your opponent’s creatures” and Cast Out should read “until Cast Out is destroyed” for exactly the same trigger condition under “normal” circumstances (i.e. when only one disabling effect occurs at a time). (I can’t emphasize enough how crucial this is for clarifying what should happen for an uninitiated player, hence the oversized bold text; having 2 cards that do the same thing with different wordings or different things with the same wording can be incredibly confusing, and this game has numerous examples of both situations.)
2. Change Hixus to read “Enable all of your opponent’s creatures that were disabled by Hixus” if Hixus shouldn’t affect Cast Out or any other (less commonly played or yet-to-be-released) disable effects.
This would reflect a categorical change in what Hixus does when any concurrent disabling effects take place, e.g. Imaginary Threats, Lunarch Inquisitors, Cast Out, and so on; this might or might not be a broader change than v2.7 actually intends.
3. Change Cast Out to read “creatures disabled by Cast Out cannot be enabled by other supports (or other creatures and supports, or other cards, etc., as the case may be)” if the intent is to have Cast Out represent a “stronger” disabling effect than Hixus, so that it will override Hixus’ enabling clause (and perhaps other cards’ enabling clauses as well) without affecting most other interactions more broadly than intended.
On the surface, this seems similar to the intended adjustment, but this method would avoid the clumsiness of dealing with each new interaction on a case-by-case basis and would provide a consistent template for any similar situations that may arise in the future (or that have already arisen but haven’t attracted the same amount of attention as Hixus and Cast Out specifically). One of the advantages of this approach is that it would help with reporting bugs, as the wording on each card would clarify the intended behavior when multiple disabling/enabling effects interacted together.
4. Either change Hixus to read “Enable all of your opponent’s creatures, unless those creatures are also disabled by Cast Out” or change Cast Out to read “Creatures disabled by Cast Out cannot be enabled by Hixus when Hixus is destroyed” if the intent is to change only the interaction between Hixus and Cast out specifically, without altering any other interactions or clarifying the broader precedent.
This is the narrowest and most awkward adjustment, but if the current functionality is going to change without making the rules more transparent in general, then Hixus will no longer do what it says it does (namely enable the opponent’s creatures when it’s destroyed, independent of any other conditions), and one or both of the card texts in question should be brought in line with the new interpretation. From the standpoint of making the game more intuitive, this might make sense, as most players initially seem to find the current behavior surprising and many tend to interpret it as a bug, but as written, what happens now is also most likely what should be happening logically, so a rules change, however minor, should be treated differently from a bug fix as far as card texts are concerned.
This also potentially touches on a more fundamental templating distinction between continuous slot-disabling effects (e.g. Desert’s Hold, Suppression Bonds, and Claustrophobia) and temporary or creature-specific ones (e.g. Hixus, Deadlock Trap, and Skyline Cascade) which isn’t clear at all based on the current wording and in-game rules tooltips available on various cards (even the “Disable” keyword itself varies between showing up in plain text, showing up in bold text with no tooltip, and showing up in bold underlined text with a tooltip), so adjusting which template Cast Out adheres to and/or clarifying the in-game language used to differentiate between a creature and a creature slot might be relevant somehow as well.
Regardless of how the Hixus/Cast Out change is implemented, confusion along these lines tends to highlight some of the ambiguous language and inconsistent templating that sometimes pervade this game; given the turbulence of the last year or so and the transition from one developer team to another, leaving some of the ambiguity unaddressed until now is understandable, but at this point, a comprehensive rules and templating overhaul is sorely needed in the long run in order to preserve even a basic level of accessibility as far as complexity is concerned (among other sources, Mark Rosewater, the head designer for paper MtG, touches on that issue in this article and this blog post; while some of the specifics might differ in MtGPQ, similar principles would apply in terms of ensuring players can realistically understand what their cards do and can reliably distinguish between bugs and features on a regular basis).
Hopefully future releases will tend to emphasize improving consistency and clarity for newer (or ideally all) players; in the meantime, it’ll be interesting to see what Dominaria holds in store for the new Standard environment, including whether it has any new disabling supports of its own that might have helped to inspire the upcoming changes.