I think an interesting addition of content could be achieved by improving the Illusion Dungeon concept. Making dungeons with the purpose of leveling and obtaining intermediate gear as a reward.
Currently NovaRO is mainly using the Gramps system for leveling. TBH I think the current Gramps system is very well done for what it was designed for and how it achieves the design goal so I wouldn't necessarily suggest changes to that system.
However I feel like the gap between Eden gear and Mora, Thanatos, Temporals and GSS, GMT, etc is not well structured (which is KRO's fault). I feel like the jumps in difficulty and rewards can be awkward to work through, because the progression doesn't seem very smooth(the way in which effort/reward ratio progresses is weird). I think finding the proper balance between linear and exponential progression is just difficult to find which lead to this problem. The way in which the difficulty increases as you progress through content could be improved upon and I can see what KRO was going for with the Illusion Dungeons.
However with the current state of Gramps and Illusion Dungeons I feel like they are somewhat clashing against each other. While Gramps is a pure leveling system the important aspect of it is the extremely high efficiency, it allows you to skip most of the usual grind and intermediate steps between gear progression. Because the max level can be obtained so easily the usual intermediary steps are removed since it wouldn't make sense to farm for subpar gear when basic gear is enough to grind gramps and from there proceed straight into high- or endlevel gears. This completely removes the necessity for two of the main purposes Illusion Dungeons were designed for (leveling and intermediary gear).
However I don't feel like removing Gramps would be an appropriate solution. Instead I think it's worth thinking of ways the Illusion Dungeons could be improved to achieve a reasonable effort/reward ratio which makes it efficient to farm them in between Gramps stages.
Another idea which I feel worth thinking about could be combining Illusion Dungeon concepts and rewards with the Gramps system.
This is a rather complicated set of systems to work through though. I'm not sure if there is a way to change the way leveling and gear progression work in NovaRO without making significant changes.
TLDR: Making some system which is slower than the current Gramps, but a little better by the addition of intermediary gear rewards could make the whole progression a little smoother maybe.