CAs are definitely not the way to go. They already have some clue benefits and many other purposes, and ties a very casual activity to very high end PVM. Yes, combat is a lot of the source of clues, but it doesn't truly define the activity.
There are three lines to me, all with various benefits and downsides.
- Achievement Diaries, scaling with individual completed tiers. This does have the notable effect that pures and other limited accounts may have very limited access to upgrades, especially if elite clues and master clues are tied specifically to high tier diaries.
- Scaling it with clues completed. This is the most self-reinforcing way to do it, but might discourage people from getting started especially on second accounts, and doesn't allow for the kind of interconnectivity that progression in runescape is so well known for, doing different things to further a goal by allowing increased efficiency.
- Collection log slots. This one is personally my favorite. It gives a lot of advancement for just playing the game as you like, is still very self-reinforcing since a large section of the collection log is in clues, encourages doing all tiers of clues to diversify collection slot gains. Especially with the proposed tiers of collection log status, that ties in extremely well.
Edit: Fair few people are saying that collection logs should just not have tangible benefits. I can see that, yeah. It's definitely a strong precedent to set and I understand that that direction is one people are against.
Someone also mentioned quest points, which i do find rather neat honestly. I'd probably make that a fourth point on the list. It has less gating complication than diaries, since the high end for quests is a fair bit lower and quest points are less mutable and tiered than the various levels of diaries.