I propose the following rules change to improve racial variety in perpetual leagues, not to mention bringing about World Peace

"Only one of the 3 skills Claw, Mighty Blow and Piling On may be used by a player in resolution of a knockdown. Which skill to use is at the discretion of the owning player's coach."
First some numbers:
1. REFERENCE POINTS
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(a) Crowd surf or simple foul: KO+ 42%, CAS 17% [=unchanged]
(b) Foul with Dirty Player: KO+ 58%, CAS 28% [=unchanged]
(c) Knockdown damage by rookie teams: (various) [=unchanged]
2. MAXIMUM KNOCKDOWN DAMAGE IN CRP vs. AV7/8/9
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(d) Stun+ 83%, KO+ 58%, CAS 31+
3. MAXIMUM KNOCKDOWN DAMAGE AS PROPOSED
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(e) vs. AV7: Stun+ 76%, KO+ 42%, CAS 18%
(f) vs. AV8: Stun+ 58%, KO+ 34%, CAS 15%
(g) vs. AV9: Stun+ 51%, KO+ 25%, CAS 11%
Note that the skills still work in combination, in the right circumstances. But the effect is more additive than multiplicative. And there is a law of diminishing returns as regards the stacking of skills (but each skill and skill combo is still valuable in the right circumstances). Each skill retains its character even in combination.
Injuries taken by knocked down AV9/8/7 players with no damage skills used is in the proportion 6:10:15. Under the proposed rules change, the maximum knockdown injury possible will change to a proportion of approximately 6:8:10 (flatter, but not completely flat as in the CRP). I suggest this is about enough to curb the AV9 teams, but without making the Claw teams dominant (either by number or by style). Somewhat better skill retention is implied for all teams. Killer teams would still be valid IMO, but would perhaps not be seen as a default option (more skill would be required in both play and development, SPP acquisition would not be quite as fast).
Many benefits would flow from this, I suggest, not just racial variety, but also development and tactical variety. I could go on, but I think this post is already quite long enough.
[Edit: for the boffins, please note that the above numbers are not cumulative, since it depends whether you are aiming for a KO or a CAS (when using Piling On).]
Comments most welcome.