This isn't highlighted in the rules, but it was deliberate. When I worked in the corporate world, I saw too many people who were elevated beyond their abilities to function effectively. They made a big show, they attracted big clients, but they just couldn't handle the daily activities that made those clients happy. Thus they relied on the people around them, and had to make judgement calls about who would be best able to do this. On the surface, they'd look at people on the same level as them, people who talked a big game but didn't know what they were really doing either. If they looked deeper, they'd see those other people in the office who were kept around because they didn't seek the limelight, and who were incredibly effective at working behind the scenes. Of course then there were the people in the office who had reached the middling levels with an incredible range of skills and contacts at their disposal... everyone within the company knew that they were thd true backbone of the enterprise, often kerping things running smoothly for decades while the young high fliers crashed and burned after a few years when they made one too many mistakes for biting off more than they could chew.
It's hard to do something similar to this with a flat level system, where it's assumed that everything goes up together (even if some parts of the character advance more quickly with each level due to the class specialties).
In The Law, it's assummed that characters will gain some kind of advantage or xp boost every game or two at first, then gradually less as they gain power. They are also specifically capped in their Agency Rank, by not allowing it to be higher than the character's highest attribute die. This means that after their first session, they could easily ascend from a rookie (d4) to a full agent (d6), and in a game or two could boost their rank a second time (to d8), but this incurs the higher difficulties for everyone in the team, so other agents might not be too happy with this. Getting to the higher dice (d10 and d12) will probably require waiting until an attribute is boosted, but if an agent has avoided any attribute reductions due to permanent in-game effects, it could still be feasible to reach such heights after a dozen games or so... it's a pretty rapid development compared to the long slog of D&D campaigns lasting years or even decades, but it has that built in feature that if a character reaches that level of heroic notoriety without the attributes to effect the people around them, or the defences to avoid the incoming effects launched against them, they'll crash hard.
In this new game about the spirit world, I'm not necessary going to limit the equivalent Rank die to never being higher than the highest attribute die, but I suspect that this is how things will work out by default. Characters will still pass the same advancement thresholds that they do in The Law, but they'll only be able to increase their rank once they've completed a story. At the beginning, a story should only take one or two game sessions to resolve, but as a character gains power, the more complicated stories will take longer.
It's the pacing of these later stories that I'm worried about at the moment.
(The following ideas are based on the number of story beats that would need to be hit, as a character moves through their tale)
I like the triangular number system...
...something like it is already built into the advancement thresholds in The Law.
What I described in earlier posts goes a bit like this. Each level requires a number of story beats equal to the level number before it is completed. The overall number of story beats follows the progression. So, if a character is working through level 1, they haven't achieved a single beat yet...while working through level 2 occurs after that first beat has been resolved, and as the character progresses through their second and third beats...level three follows through beats 4 to 6... etc. But this is feeling like a rapid acceleration from lowly sprite to epic godhood.
In playtesting, I've found that the d4 rank die of starting characters is a bit problematic, but it does mean that characters have an innate desire to mitigate the issues of the low die by working together (which is always a good thing), independence only really becomes viable if you've got a decent rank die, some useful equipment, or a situational advantage (and it helps more if you've got at least two of those). So we need to get characters above that d4 level quickly, but don't necessarily want them ascending to the d10 and d12 quickly.
It might slow things down at higher levels to require triangular progression within levels rather than overall. In such a system, rank 1 would still only require a single beat, level two would require (1+2=) 3 beats, for a cumulative total of 4. Level three would require (1+2+3=) 6 beats, for a cumulative total of 10. Level four would require (1+2+3+4=) 10 beats, for a cumulative total of 20. Level five would require (1+2+3+4+5=) 15 beats, for a cumulative total of 35. If we assume a beat is hit every game or two, then it would take 35-70 games (an average between these games would be 52, which happily coincides with a year of weekly solid game play).
...doubling gives us an faster progression through the numbers until we get to thst last level, when things really decelerate. I think I'm happier with that triangular progreesion within each of the levels.
The only problem I've got nagging at the back of my mind is that it's all feeling a bit abstract and artificial.
So that leaves me thinking about other solutions... but that will be the next post.