I am assuming that by the prominance of Instosis that the most powerful combo with SoR is with Mitosis. There are others: Pharaoh, Eternity, but none have proven to be particularly overpowered. The previous SoR was also evidently UP, as it was very rarely used and not very effectively.
The chance does not need to be high to nerf the OP combo. 8 uses of mitosis means that 2 messed up uses will likely ruin the OHKO. Even a 10% chance of a mutant gives a 19% chance of the combo being ruined, and a 57% chance of getting at least one use of mitosis ruined.
On the flip side of the coin, most decks using mitosis without SoR should see no big large difference. Decks like Frogtosis will not be hurt by getting a mutant once in a while - it's even sometimes a buff.
A direct fix to SoR would likely be difficult: Replacing it with the old SoR would leave it back in oblivion, giving Adrenaline has issues with the 2TTW decks, etc. The solution that I offer does much to detract from the most overpowered combo (not all of them, but the most powerful one), adds to the theme, and doesn't nerf other balanced decks.
To deal with each point seperately, instead of the fluff above:
*0 activation cost - this was UP before, no reason why it's a problem now.
*2 uses for time - The time creatures with abilities are Pharaoh, Scarab, Eternity (only applies when flying), Deja Vu (obviously not a problem), Fate Egg, and Anubis.
* Pharaoh: Producing a mass amount of scarabs has been tried. I can't seem to find the link now, but there was a thread where a player tried Mummy+RT+SoR. It worked ok, but not great (despite his claims of an 85% FG win-rate). Although this is powerful, it doesn't seem to be a particularly OP combo.
* Scarab: Only useful when scarab needs to eat 2 creatures at once. This situation is very rare, since you will normally have a swarm of scarabs anyway. Not particularly useful here.
* Eternity: Requires it to be flying, and is situational anyway. Not a problem here.
* Deja Vu: No longer works.
* Fate Egg: Not particularly powerful due to the randomness.
* Anubis: Mass quint is heavily situational.
* Stacks on the same turn: The most important part of SoR. However, to be able to spam SoRs requires some method of stalling long enough to get the combo in hand. This basically requires sundials and the like, at which point you're just better off going with Instosis. Again, decks that use SoR to mass produce Scarabs have been tried without OP success, and stacking SoR's on others are pretty clearly not very OP.
1) Your suggestion either is too unlikely to nerf the OP combo or too likely thus nerfing balanced combos.
Let's do some math:
10% chance: ~57% chance to affect 1 SoR use, ~19% chance to stop the combo
15% chance: ~77% chance to affect 1 SoR use, ~39% chance to stop the combo
20% chance: ~84% chance to affect 1 SoR use, ~50% chance to stop the combo
25% chance: ~90% chance to affect 1 SoR use, ~64% chance to stop the combo
30% chance: ~94% chance to affect 1 SoR use, ~81% chance to stop the combo
This should be enough to see that the chance does not have to be particularly high to nerf Instosis enough that it is no longer OP.
So what other decks use Mitosis?
Devtosis: A failed Mitosis use does not adversely affect this deck enough to be a serious issue, particularly when the chance is low. A mutation can even be positive for this deck, as it could add damage to the deck.
Vamptosis: A failed Mitosis use does slightly more to this deck than Devtosis, but it is still not disasterous.
Frogtosis: A failed Mitosis use does little to hurt this deck, and likely ends up helping it.
In general, Mitosis is used to add creatures in decks where a failed use of mitosis will not adversely affect the deck enough to call it UP. Mitosis is generally used on low-cost, low-powered creatures, when a mutation is just as likely to help as to hurt. Mitosis can even be splashed into rainbow decks with greater effect with this change.
To summarize:
* The chance can be as low as 20% and stop the Instosis combo 50% of the time
* The chance does not affect other decks adversely; it helps them if anything
* SoR's main target is with Mitosis - the other targets are powerful yet not OP.
* Because of this, a direct hit to SoR will likely leave it back where it was pre-buff: nowhere.
* Thematically, organisms that are born by Mitosis carry the "risk" of genetic mutations, which is the only way they can be genetically different than their parent.