Invincible as one of the Cardinal Heroes would be interesting

Mostly because of how his powers would work in the setting and the way he would react to stuff, depending on the weapon he gets chosen to wield (sword, bow, spear or shield)

I mean nothing is really stopping him from flying into Siltvelt or Shieldfreeden after the trial in the case he gets accused as the shield, without the attack restrictions in the other weapons he could be a legit menace in the fight against the waves and would be highly desirable by the other countries. Not to mention everytime he gets beaten up just makes him stronger thanks to his viltrumite DNA, he would be using the weapon skills somewhat well but mostly he would fall back to his viltrumite powers for most fights

The key thing about this idea is when in the series Mark gets summoned, because depending on the arc he is going to act differently from other parts of the series.