Diablo 4 has a clever way to turn a bad Transfiguration roll into a perfect one
All it takes is a Modifiable transfigure and one trip through the Horadric Cube's Mythic upgrade.
Diablo 4 Season 15 has a crafting interaction that turns a disappointing Transfiguration roll into a perfect one! Wudijo shared a post on his X (formerly Twitter) which shows that upgrading a Modifiable Transfigured Unique to Mythic quality can max out its Transfigured Item Quality affix after the fact.
In the example he posted, the resulting Mythic 
Eaglehorn ended up with the full +30 Item Quality, which is the maximum of the Transfiguration affix range you can have for this stat.
wudijo@wudijo𝕏HOLY, modifiable transfigured weapon upgraded to mythic maxes the transfig item quality after the fact 😅 #DiabloIV
That potentially changes what you are looking for when transfiguring a future mythic item.
Any Transfiguration Roll Could End Up Maxed
Normally, getting Item Quality from Transfiguration is only half the battle, so to speak. The affix itself can roll from +2 to +30 Item Quality on two-handed weapons (+1 to +15 on other items), so seeing Item Quality appear does not necessarily mean you got the roll you wanted.
But there is another important outcome from Transfiguration, which happens very rarely and allows the item to remain Modifiable. And that combination appears to be the jackpot!
If you get Item Quality and the Unique remains Modifiable, you can take that same item to the Horadric Cube and use Season 15's new Upgrade to Mythic recipe. The upgrade will then push the Item Quality all the way to its maximum of +30 (on two-handed weapons) on the resulting Mythic Unique. So a low roll might not be relevant at all if the item survives Transfiguration.
Anyway, it makes sense that this can happen. Blizzard's Mythic Unique rules state that the affixes added through Enchanting, Transfiguration, and Tempering always roll at their maximum values when added to a Mythic Unique. Season 15 also changed the Horadric Cube's direct Mythic upgrade into a 1:1 conversion, which means your Unique becomes the Mythic version of that same item, with its existing affixes and modifications intact.
So naturally, if you put those two mechanics together, you would get the interaction Wudijo demonstrated. And since that rule covers Transfiguration affixes in general, this is probably not limited to Item Quality. Other Transfiguration outcomes with a roll range, like the bonus damage or skill rank affixes, should get the same treatment. However, as previously mentioned, getting this result will not be easy...
The Item Still Has to Be Modifiable
You cannot simply transfigure every unique and fix it afterward. Blizzard specifically stated that Unmodifiable items cannot be used with Upgrade to Mythic and even warns players about transfiguring something they plan to upgrade. So you still need the rare outcome where your transfigured item stays Modifiable after.
Okay, but what if that happens and you got a low roll? Again, do not worry about the number. Even a poorly rolled item affix may have just become a perfect one waiting for its Mythic upgrade.