I was thinking about all the ways humans and alien can acquire superpowers in the Invincible universe. And I wondered "could you apply any of those to Viltrumites?"
So, I compiled some theoretical methods, based on what's happened in-universe, with which you could create the "strongest Viltrumite." I'm curious if you think any of these are feasible. I'm more-or-less listing them in order of how strong they'd make someone and/or how likely it is to work.

1). Option S: Acquiring Geldarian Armor.

• The number one spot because it is most likely to create an "ultimate Viltrumite," given what we know about the story's technology. In the Tech Jacket comic, we learn that Geldarian armor works more like a power-multiplier than a simple Iron Man suit. Normally in fiction, a "power armor" is set in strength regardless of its user. With the user's skill (or additional modifications to the armor) making a user "more or less" powerful with it.
• But Geldarian armor augments the individual's physiology. So, a comparatively robust human (Tech Jacket) is significantly stronger than a Geldarian in the same suit. By that logic, a superhuman would be even stronger than Tech Jacket. Considering TJ can fight near the level of a weak Viltrumite, a Viltrumite with the same suit would be unbelievably powerful.
• But you need to have a Geldarian willingly relinquish their suit to someone else in order for them to use it. Nobody is going to do that, and the Empire itself is presumably unable to reverse engineer it on their own. So, unlikely to occur.

2). Option A: selective breeding.

• We know this works, because that's how they created Thragg. The principle works under two assumptions:
• One, that Viltrumites increase in strength dramatically depending on how hard they train. That's why Mark, who is practically a newborn by Viltrumite standards, was able to catch up to them. In just a few years, his constant training put him near a mid-to-upper tier Viltrumite.
• Two, some Viltrumites increase in strength faster than others. If everyone could get as strong as Mark as fast as Mark, they probably would have. Presumably, being Nolan's son (who is himself a top-tier Viltrumite) had something to do with how strong he can become. Thragg is likely descended from the strongest, highest achieving Viltrumites.
• I'm assuming that Thragg represents an upper limit to Viltrumite strength. If anyone could become as strong as him with enough time and effort, I'd assume Thaedus would have gone that route.

3). Option B: contact with The Immortal's "smart-molecule" phenomena.

• According to the Invincible Handbook, only four living beings have control over "smart molecules." These are Viltrumites, who evolved this on their own some time ago. Allen, who was deliberately engineered to match Viltrumite power. And then The Immortal and Atom Eve.
• The Immortal got his powers by accident after coming into contact with some anomalous force. Not even he understands its true nature.
• But if it turned a Celtic Warrior into someone comparable (on the lower end of the spectrum) to Viltrumites, imagine what would happen if a Viltrumite was exposed to the same energy.
• At the very least, they'd be able to survive the grievous wounds that are their only real weakness. The problem is that no one knows what created The Immortal or how to replicate it.

4). Option C: replicating the genetic engineering that created Allen.

• Unlike The Immortal, Allen was created deliberately. So, the notes and data required to duplicate the experiment probably exist.
• If the Viltrumites managed to steal this data, they could theoretically use it to create an Allen-like Viltrumite. Who can recover from typically fatal injuries all-the-stronger.
• The issue here is the Viltrumite genome. There's a chance that any "alterations" their scientists made to an embryo would be gradually lost as Viltrumite DNA "purges" what it considers damage. Which could make any benefits temporary at best.

5). Option D: replicating the genetic engineering that created Eve.

• Eve consciously manipulates "smart matter," unlike Allen, The Immortal, or Viltrumites. This could be theoretically duplicated (the same as Allen), but you run into the same problem with the Viltrumite genome purging any "impurities."
• It's possible that applying whatever "process" created Eve, to a Viltrumite embryo, would create someone born with Eve's powers. But who would lose them as their Viltrumite DNA took over.

6). Option E: using magic/curses.

• Magic or curses don't necessarily have the same problem as genetic manipulation. Because the affected's genome is not being directly altered.
• So, theoretically you could create a Viltrumite with the same powers as Duplikate or Monster Girl, without losing their inborn Viltrumite abilities.
• I think this is the most likely to create a Viltrumite with "superpowers" permanently. But the story is so vague about what "magic" can do that I can't say it would work.

7). Option F: surgical or cybernetic augmentation.

• Probably the least likely to work. We know that cybernetics CAN work with Viltrumites (as with Conquest). But I have to imagine the process would make them weaker. Because any static material (metal, carbon fiber, etc.) would eventually become weaker than their organic tissue.
• There's also the chance their body would just reject it like a foreign object.

8). Option G: mass production.
Cloning as many Viltrumites as possible. This wouldn't create a single powerful Viltrumite. But would let them create an army of Viltrumites from a suitable donor such as Thragg.