There are many possible words based on the meaning of "plain" that is used.
The word pretty used a woman's appearance could be the opposite of plain.
The word peak used for a mountain would be the opposite of a plain or prairie.
The words pretentious, pompous, or poshcould be the opposite of a plain or ordinary event.
The word private could be the opposite of something plain, as in obvious.
The word phenomenal could be the opposite of a plain or mundane experience.
The word prodigious could be the opposite of a plain performance.
polar
Polar is a modifier to opposite, meaning it adds emphasis to it. There isn't a word that is a synonym for opposite that starts with a 'P'.
Alluvium
starts
Reversal is a synonym for the word switch. It begins with the letter r.
Asinine is a synonym for moronic. It begins with the letter a.
Marvelous is a synonym for good. It begins with the letter m.
Consumed is a synonym for tired. It begins with the letter c.
Error is a synonym for not true. It begins with the letter E.
Eternally is a synonym for forever. It begins with the letter e.
A synonym for mother that begins with R is relative.
Alternate is a synonym for change or vary. It begins with the letter a.
Bravery.
Energetic is a synonym for driving. It begins with the letter E.