Okay, I think this is the way I'm going to start using SIEGE. As usual, it seems 100x more complicated to explain in words than it does in practice, but I'll give it a shot...
1. One Challenge Base (CB): 16 that gets better by 1 every other level
2. Select 3 Prime (+3), 2 Secondary (+0), and 1 Non-Prime (-3) attribute at creation.
This breaks up the monotony of good/terrible checks and also makes more rolls possible.
3. Challenge Levels (CLs) are now determined as follows:
Easy: -3, Average: 0, Hard: 3
4. Write your Challenge Classs (CCs) for each attribute (CB - Prime Bonus - Attribute Bonus + CL). These are the numbers you need to beat to succeed.
For Example, a Lv6 Fighter with a Prime Str stat of 16 (+2) would have CCs 5/8/11.
(13 (CB) - 3 (Prime) - 2 (Str) + each CL)
So let's say he needs to make a Str check to bash a door down. The CK decides that's a pretty Easy task since the door is old and wooden.
He rolls a d20 and gets 9. He then just looks at his sheet and says, "I have an Average success".
CK lets him know he succeeds.
Next thing to tackle is Saves. You still compare level, but now only increments of 3 matter. So a LV1 Fighter Saving vs a 4 HD creature has to make a Hard check. If the creature were 3 HD, then the Save would be an Average check.
Last mechanic to add is Advantage/Disadvantage in situation where it makes sense.
Two ways to do that:
Like 5E or just assign different difficulties to different characters. For instance, if the Ranger wanted to track it might be an Average challenge, but for the Cleric it would be Hard.
Note: CLs could keep climbing if needed. Heroic: 6, Legendary: 9 for instance. But since 90% checks will be Easy - Hard, that's what goes on the sheet!