I really like this! It would work quite perfectly terms of both the picture and symbolism.
I'm still a little torn about the Magician. Bekkler works, of course, and I'd like to fit him into a suit card at least since he's a great character, but my preference would be for the trump to be taken up by a more significant presence. I do think Crono could function nicely, but I admit that aesthetically it's a stretch.
I can see the issue - he does provide the clones etc., but he's more of a minor character than a major one. Still, he's the best fit as the magician. The only alternative I can come up with would be Spekkio, since he teaches you magic. On the other hand, the Magician brings his power down to earth, while Spekkio remains somewhat detached. Also, Spekkio could be the King of Wands, assuming that he's not assigned to Strength.
We seem to agree on these choices! I admit that I'm especially attached to Ayla as Strength -- aside from the obvious correlations, the image of her with a beast would be especially fitting -- and Robo as the Hanged Man, for these reasons.
- The Hanged Man has connotations of traitorship. This fits Robo's relationship with Mother Brain and the other robots.
- The Hanged Man is also in a strange, inverted position due to his desire to seek knowledge. This fits his relationship with humans: he doesn't fit in, but is eager to learn about them. He willingly chose that role, as the Hanged Man does.
- Robo selflessly helps cultivate Fiona's Forest and in the process, gains mystical knowledge, evidenced by the Entity conversation. That's very Hanged Man, to me.
- There's also the possible meaning of sacrifice, which aligns with Robo's identification with Prometheus and his later sacrifice in Cross.
- Robo is a badass and so is Odin. (This is the least effective of my arguments, I confess. 
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All in all I like how there is something tragic and selfless about the Hanged Man, and how that speaks to Robo's willingness to go into the uncertain future, and of course his eventual demise by way of FATE.
All of these, save the traitorship aspect, also fit Gaspar, however. I am not entirely sold on the appropriateness of the Hierophant to Belthasar: the role definitely fits the one Belthasar filled in Zeal, and the one he fills in Cross, but the most striking image we get of Belthasar is definitely as a solitary madman. Still, I feel that Belthasar could fit as the Hierophant regardless, and if we made Robo the Hanged Man we'd have to figure out where Gaspar fits.
And Magus as the Devil is a given, for me. Too perfect. I also liked the fact that Lucca gains her best equipment through the Sun Stone; there's an added little detail.
Indeed, Ayla taming the beast would work very well. After all, the Strength card shows a woman.
Another point in favor of Robo being the Hanged Man is that after the party fixes him up, he reaches a philosophical crisis - what should he do next? He wasn't programmed to have his own free will. So, at least for a time, he is suspended between taking orders as a robot and forging his own identity.
As for Magus, there's no question about that matchup.
Re: Lucca and Sun Stone, I'll address that in the last paragraph.
The Gurus is where we have more difficulty. Most of the Hanged Man points do indeed fit for Gaspar. Even the traitor connotations may be met by his helping the party against Queen Zeal, although that is a stretch. If we change things around, I'm not sure where else he can be assigned besides the Hermit, which is a less ideal match (though not a mismatch).
If we include Gurus, I'd rather all 3 be used, just like I'd rather have all 7 party members than half of them. Melchior fits well as Temperance, a card that emphasizes moderation and transformation. He's the Guru of Life, and he fixes/tempers the Masamune as well as creating other powerful items. In combining a broken sword with dreamstone, he takes two things and creates a stronger, unified third. I can't think of a proper replacement if we use none of the Gurus.
I can't see Belthasar as the Hierophant. I've never played Chrono Cross, so I can't comment on that. But the Hierophant emphasizes community and maybe even conformity, and Belthasars solitude runs counter to this. The Hierophant is about stable structure, while Belthasar loses his mind. I honestly believe that Mother Brain is the best fit.
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I agree with your options regarding the Moon card. I chose the Black Dream myself because the meaning fit (as you described) and because in terms of the Trumps as the Fool's Journey, the Moon is often perceived to fulfill a "dark night of the soul" sort of function -- the period of illusions and terrors, the "valley of the shadow of death" where the Fool must traverse before reaching the Sun. I feel that the Black Dream may color the card a touch negatively, so I've no quarrel with either that choice or Dreamstone.
Indeed. The choice may come down to getting everything to fit together and not use duplicates. For example, we might really want to use Dreamstone as an ace of pentacles, although that's not finalized by any means. Or maybe we'll want to place the Black Omen in the minor arcana, though I'm not sure how that would play out.
That's similar to the reasons I placed Lavos as Judgment, because aside from the obvious apocalyptic associations (which are traditional depictions of the card), having Lavos next to the end of the suit striked me as very appropriate, since you would encounter it just before countering the Entity-Nu-World trump. But again, Death fits Lavos extremely well, and it could swap places with the Chrono Trigger with no problems.
The order of the cards does matter, although it makes this even more complicated. Ideally, we should aim to have the order consistent with logic and the storyline where there is room to do so.
I feel that Judgement: Chrono Trigger, and Death: Lavos fit much better than the other way around. The Judgement card is about resurrection, facing the past honestly and making major amends. Likewise, the Time Egg brings back Chrono, opening the final phase of the game. It also takes the party back to possibly the most painful point in their adventure, and allows them to make amends. Could Judgement be Lavos? maybe. There are the apocalyptic connotations, and he certainly passes his judgement on the excesses of Zeal. But his cataclysmic destruction seems a better fit for Death, and I outlined other reasons previously. Then there's the issue of assigning the Chrono Trigger to Death, which seems an ill fit at best - the Time Egg is about potential and making amends, rather than destruction.
I can see the sense of Lavos coming right before Nu/Entity. But taking storyline into account, we come across Lavos fairly early on, and first confront him directly some time before the end. The Chrono Trigger, on the other hand, comes right near the end.
Regarding the Star, again, your reasoning for the Pendant is quite solid. Since Marle is the carrier of the pendant, though, I feel as though a combination of our two ideas may be possible -- especially since Marle acts as the impetus for the journey to begin, and is a symbol of faith and hope when Crono dies, and since the card is traditionally depicted with a woman anyway.
Indeed, we could have Marle wearing her Pendant as the illustration regardless of which one we choose.
Likewise, we could combine Lucca and the Sun Stone, perhaps with both of them in the illustration. and/or Lucca holding the Wondershot, with a ball of golden energy at the muzzle.
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if anyone else has ideas, feel free to contribute!
