***SPOILERS*** Possible Identity of the 5.5 trial might be different then we expect.

[Nezerius]

Or here's an even crazier idea: why not have more than one trial for Patch 5.5x?
We did get three trials back in Patch 2.5x (Gilgamesh, Nabriales and Vishap) and two trials back in Patch 4.5x (Seiryu and Yojimbo) after all...

To add to this, they also added two new trials in patch 5.2. Ruby Weapon (5.2), and Memoria Misera (5.25). So it definitely wouldn't be out of the ordinary for SE to introduce two trials in a patch.
Another thing to keep in mind: the x.5 patches always get split up into two parts, so it's definitely possible for them to throw Diamond Weapon into part 1, and a MSQ trial into part 2.

[Vyrerus]

I hope y'all are right, and that we get two trials.

[Cilia]

Things are complicated here.

On the Werlyt front, we know Diamond Weapon is coming with Zenos as the loaded data. However there are strong intimations the G-Warrior is going to see more use, and even after Diamond Weapon is dealt with Valens is still going to need to be dealt with. One can only guess how this puzzle is going to fit together.

On the MSQ front, we've never had a full-on X.5(5) trial and Estinien's (intimated) involvement in dealing with Lunar Bahamut would put things closer to a solo duty (typical for X.5(5) patches). Then again, curveballs can be thrown.

... and with the general shakeup of developed content, and Bozja still needing to be accounted for, how things are going to play out is pretty much anyone's guess.

[Nezerius]

On the MSQ front, we've never had a full-on X.5(5) trial and Estinien's (intimated) involvement in dealing with Lunar Bahamut would put things closer to a solo duty (typical for X.5(5) patches). Then again, curveballs can be thrown.

Let's not forget about patch 2.5 adding The Chrysalis, followed by the Steps of Faith in patch 2.55. Both were trials tied to the MSQ.

[Raikai]

Why not both though?

I get your point, in the Mech being used to combat Diamond, but on that, I kind of picture it being used to weaken Diamond, so we can fight it in an actual Trial.

[Seraphor]

I don't think we'll be fighting Zenos as the Oversoul. We see a test being conducted and the consequence is Valens deducing that Zenos' data is much too unstable and unreliable due to his nature as a ravenous beast.

He's likely to use the data of someone who he thinks is easier to control. As for who that could be? Maybe Solus or Varis. Or we might even see the data of some currently unknown Garlean soldier to give some insight to one of the Legions that hasn't been touched on as of yet.

I suppose we can't rule out Valens himself powering himself up somehow and being a 'surprise' fight. Though I'm inclined to think he'll probably be eliminated by Gaius and/or whichever of the orphans happen to survive.

A Sephiroth van Gast perhaps. What other obedient supersoldier do we have to draw from to conclude this FF7 themed trial series?

[Kesey]

A Sephiroth van Gast perhaps. What other obedient supersoldier do we have to draw from to conclude this FF7 themed trial series?

Who wants Sephiroth when we have his clearly-ripped-off-but-way-less-interesting wannabe Zenos?

[Cilia]

While 2.5 / 2.55 did have two trials, neither had an EX difficulty. That's highly irregular to what we've been given since; I'm not saying it's impossible we'll get 2 full Trials (with an EX mode for Diamond Weapon, obvs) just that it's not likely based on available data of what's been implemented in previous X.5(5) patches.

Valens flat-out stated he was using Zenos' data for Diamond Weapon; I don't think they'd make a shocking swerve for the sake of unpredictability and it makes in-universe sense (with Zenos being one of if not the finest warrior ever produced by the Empire). I agree it would be nice to learn more about other Legions (and question the wisdom of using the combat data of people we've already defeated against us), but unpredictability and surprises are not this game's strong suit for better or worse.

Could we get Trials against Lunar Bahamut, Diamond Weapon, and Valens van Varro? Well, yes, we could. However between that high Trial load and 5.X's other loose ends that need to be tied up (YoRHa: Dark Apocalypse's conclusion and what should be the last segment of Save the Queen), along with the obligatory dungeon and MSQ content, based on what we've gotten in Shadowbringers so far I have a very hard time believing we're getting two dungeons (MSQ / Save the Queen), a raid, an open field instance, and three full Trials.

And where's my goddamn crafted GNB weapons?! Exarchic stuff is cool and all but it's too expensive to buy a full set on my alt. I want a chocolate bar, don't touch me...

[Seraphor]

Valens flat-out stated he was using Zenos' data for Diamond Weapon

I think most people interpret that scene as him flat out stating is isn't going to use Zenos.

If they were going to use Zenos, Emerald would have been a much better choice. They could have given his oversoul a Shinryu reprieve, complete with it's green glow, and that way the Diamond-Gaius reveal in 5.5 would have been much more meaningful and a fitting conclusion to the 'Werlyt' saga, ending it with a fight in Werlyt itself.

Now, they have to top the Gaius/Werlyt battle somehow.

[Cilia]

I think most people interpret that scene as him flat out stating is isn't going to use Zenos.

If they were going to use Zenos, Emerald would have been a much better choice. They could have given his oversoul a Shinryu reprieve, complete with it's green glow, and that way the Diamond-Gaius reveal in 5.5 would have been much more meaningful and a fitting conclusion to the 'Werlyt' saga, ending it with a fight in Werlyt itself.

Now, they have to top the Gaius/Werlyt battle somehow.

My takeaway from that scene was not "Valens is not going to use Zenos' data" but "Valens is going to keep making adjustments and throwing "savages" into the meatgrinder until he gets his desired results, e.g. a "Zenos" that is loyal and obedient to him."

I agree that using Gaius for the final battle would be thematically fitting, but apparently that's not the direction they decided to go in. I earlier said surprises are not this game's strong suit, but I was earnestly surprised they did not save him for last.

No disrespect but "They should have done [X] instead of [Y]" isn't very productive for making guesses (educated or otherwise) as to where a given plot is going.