This may be a bit of a rambling post, but several concepts all tie together, so I figured I'd work through them in a public forum... isn't that what we all do?
I've been considering level caps for progression, a la D+D 1E. You cap your progression at level 6; you can go beyond level 6, but a number of things freeze at that point (i.e. you don't get any more dice rank progressions in your attributes; you only get 1-2 additional trait points per level; you only get a 1-2 point bump on your hits every level). I want to keep the numbers relatively small overall, and this seems a good way to do it. Level escalation is a relatively modern gaming concept, whereas 'back in the day', level 9 in D+D was POWERFUL. The difference between a level 9 fighter and a level 12 fighter was a moderate attack bonus and a handful of additional hit points. Your level 9 fighter had a better than decent shot one-on-one of taking out a character several levels higher than he was, because relative progression slowed to a crawl (excepting for spell casters...)
Mortal insects are limited to progression through level 6, and several caps kick in at that point. This is both old school (which I like) and clean (which I like). I've read about people who play D+D forever just using the Red Box with level 3 as the cap, and they do just fine. One of the charms of the original B-series modules was how they viewed level 4 or 5 as some incredible feat beyond mortal understanding... it's a nice conceit.
This limiting then ties to both Traits and Saves. Since Traits are capped at your level (excepting the Favored Trait of your specialty, which is capped at level +1), this means that no Trait ever scales better than +7 under any circumstance. This I like, because it's important that bonuses/modifier never scale past the dice; rolling the die should always be important.
And this ties back to Saves (See? I told you it would ramble)... the game currently has your Save modifier set at half your level (rounded up). Initial playtesting (and Hedgehobbit!) have shown that, since Saves will not scale with attacks (in general), you are going to hit more often as you level. This isn't necessarily a bad thing, but it does mean that the game doesn't scale equitably. That's a problem.
There are two fixes for this:
The first fix is to have Saves scale at level, so that you cap at +6 at level 6, which is when everyone would cap their attacks. Odds are good that you are going to be within a 1-2 point bonus of the opposing Save of foes you face, meaning that the modifiers will often be near a wash, and the dice stay important. This is very clean.
The second fix (which I'm leaning away from, but which I want to at least consider in the short term) is to make Saves a Trait which you can elect to upgrade as you will. This comes down to resource allocation, and means that you have to give something up in order to progress in your defensive/resistant stat. I like the CONCEPT of resource allocation as an important consideration in character building, but the two flaws here are that a) no character can really afford to skimp on their Save bonus, because it's too important and b) it's so important, that it might outstrip all other traits in overall value. If everyone is going to automatically purchase Save at their level anyway by default, I may as well build this progression into the system innately. A choice you have to make isn't really a choice.