Honestly, I think the biggest hurtle that would need to be overcome when making an ultimate from a trial series is what weapon model to use as the base for the weapons as each trial, each boss has their own unique weapons. Four Lords might be kinda easy because they could skip the boss weapons, and use the 'Crimson Lotus' weapons that dropped from dungeons, and make them glow Arimatama red or something, but it might be satisfying because some of the weapons from the actual bosses do look kinda cool.