I think Mortis trials is a great adittion to the game, however, I believe it is yet to achieve its full potential, so here are a few ideas to make the game mode even more fun:
1-More maps
2-Endles mode with modifiers that would change every few waves, when the modifier is due to change we could make a vote if we want to leave or stay
3-Unique encounters, since this is sefonis mind, you could use this oportunity to face enemies that normally would be outside our characters power levels like maybe in round 12 suddenly no enemies appear and we have to face a death guard marine or 2 tyranid warriors, maybe suddenly 1 wave instead of pox walkers we have hormagaunts, posibilities are endless