I understand your 15 argument, now here is mine. 15s is the right answer, period. A 10 man would need to recruit about 5 people. A 25 man would be able to split their raid into 2 raid teams and also recruit about 5 people. It is evenly spread that way.
If Mythic requires a certain acheter wow gold class for an encounter so be it. The answer can be "we told you about this a long time ago so go get yourself a mage or someone grab an alt mage".
Alternately, have you considered simply keeping normal as normal, heroic as heroic, and tune Mythic to be above heroic, bordering on impossible? Give all these "elite" raiders a real challenge. Make it 20 because most 10 mans would never see it and the ones that could will have no trouble recruiting.
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