From the Mind Blank spell description:
The spell even foils wish spells and spells or effects of similar power used to affect the target’s mind or to gain information about the target.
From the Feeblemind spell description:
You blast the mind of a creature that you can see within range, attempting to shatter its intellect and personality.
(emphasis mine)
Remember that there is no flavour text in 5e spells so Feeblemind really does blast a target's mind.
Feeblemind is of inferior power to Wish (since Wish can recreate Feeblemind and is one level higher) and Feeblemind clearly affects the target's mind, so it seems to me like Mind Blank would cancel all of the Feeblemind effects altogether.
The rules do not make this clear, so it's up to the DM.
Glibness protects you from magic that detects whether you are telling the truth, and the Mastermind's Soul of Deceit feature does the same and also prevents you from being magically compelled to tell the truth, so the essential questions we need to answer are:
- Does Zone of Truth detect whether you are telling the truth?
- Does Zone of Truth compel you to tell the truth?
Let's also look at whether there are any relevant rulings from the game's designers, and then think about which answers to these questions would be most fun for the game.
Does Zone of Truth detect whether you are telling the truth?
The spell does not alert the caster or anyone else to whether the speaker is telling the truth. So the DM could rule that since the spell is not "magic that would determine if you are telling the truth", and doesn't "indicate" whether you are being truthful, Zone of Truth does not interact with the anti-lie-detection features of Glibness and Soul of Deceit in any way.
On the other hand, The DM could rule that while under the effects of Glibness, or having the Soul of Deceit feature, the Zone of Truth spell itself cannot tell whether you are lying or telling the truth and thus cannot have any effect on you even if you fail your saving throw.
Does Zone of Truth compel you to tell the truth?
On one hand, the DM could rule that Zone of Truth only prevents you telling a lie, but does not compel you to speak. Since it doesn't compel you to speak, it doesn't "compel you to tell the truth". Contrast with Suggestion, for example, which could absolutely be used to compel a target to tell the truth, and Soul of Deceit unambiguously defeats that.
On the other hand, the DM could rule that since Zone of Truth prevents you from lying, you must tell the truth if you speak at all. Therefore, it is in fact "compelling you to tell the truth" and Soul of Deceit defeats it.
Relevant ruling from the game's designers
There is a relevant unofficial ruling on Twitter by Jeremy Crawford (the lead designer of the 5th edition D&D rules) about the Ring of Mind Shielding:
A ring of mind shielding and a zone of truth have no effect on each other.
From the Ring of Mind Shielding entry in the DMG (emphasis mine):
While wearing this ring, you are immune to magic that allows other creatures to read your thoughts, determine whether you are lying, know your alignment, or know your creature type.
This creates a strong case that the anti-lie-detection features of Glibness and Soul of Deceit are in fact not designed to defeat Zone of Truth.
As a DM, you might interpret this to mean that a Ring of Mind Shielding doesn't interact with a Zone of Truth because the ring specifically protects against magic that allows other creatures to determine whether you are lying, and so you could rule that Glibness and Soul of Deceit would still make you immune to Zone of Truth.
What things in the game unambiguously affect Zone of Truth? What other lie-detection features exist in the game?
I'm not aware of any magic in the published materials that directly allows a creature to determine whether another creature is lying or telling the truth, nor any magic that would allow you to uncontroversially tell a lie within a Zone of Truth, short of a Wish.
So the question sort of becomes: if these abilities can't defeat Zone of Truth, what are they even for?
Rule of cool
I know you're not asking for opinions, but I certainly think it's a more interesting and dramatic ruling if Glibness and Soul of Deceit (and the Ring of Mind Shielding) can defeat a Zone of Truth. Particularly since Glibness is an 8th level spell and the Mastermind with Soul of Deceit has to be at least 17th level. It would be weird for these powerful abilities to be circumvented by a technicality of a 2nd-level spell like Zone of Truth.
Best Answer
Your players have a great many options, given that you are open to homebrew material:
(Basically, any time you're going 'I wanna do a thing/provide my players an opportunity to become able to do a thing' and it's not easily done via spells/magic items, check out the monster manual. Most monster traits are presented in such a way as to make it fictionally reasonable for players to gain access to that trait via some sort of interaction with the monster, be that murdering it and harvesting parts of its corpse or training under it for an extended time period or performing quests for some divine patron or eldritch being or another)
Additionally, in previous editions spell research rules existed, and the ones from AD&D 2e are pretty consistent with the 5e ethos. For some reason (probably game balance-- spell creation in D&D has always been hugely difficult for GMs to properly adjudicate) 5e lacks spell creation rules, but you could certainly import or modify ones from elsewhere. A spell similar to 3.5's False Vision, which has no 5e counterpart yet is not particularly problematic would be a reasonable addition.
I have added False Vision as a 4th level illusion spell with the ritual tag for a wizard in 5e before. I had it require concentration, last up to 24 hours, and require an action to alter the image viewed. I also added a 6th level version that took an hour to cast and didn't require concentration on a round as long as the caster used no movement that round. I changed the material component to a focus component for each. Both worked fine.
Alternatively, a variant on one of the spells that protects or disguises against detection spells could be a decent starting place; they almost do what you want as-is, it would seem.