もしかしたら

1. 1Perhaps or possibly; signals that the statement that follows is an uncertain guess.Ex. 1
2. 2Maybe; introduces a speculation about a situation the speaker cannot confirm, such as whether something has already happened.Ex. 2

Neutral in register and common in both conversation and writing when the speaker wants to soften a guess or flag it as uncertain. It works like an advance signal that a statement of possibility is coming, so it naturally pairs with かもしれない rather than with expressions of strong confidence. Compared with どうやら, it does not imply that the guess is based on evidence.

• 1.
  かもしれないN4「かもしれない」 attaches after a predicate at the end of the sentence and expresses conjecture on its own; 「もしかしたら」 is an adverb placed at the start of the sentence or before the predicate, and generally needs to pair with 「かもしれない」 or similar. The two are often used together and cannot replace each other.
• 2.
  だろうN5「だろう」 attaches at the end of the sentence and expresses conjecture with fairly high confidence, and can also be said with a rising intonation to seek confirmation; 「もしかしたら」 is an adverb expressing low confidence and usually pairs with 「かもしれない」. They differ in part of speech and are not interchangeable.
• 3.
  ような気がするN3「ような気がする」 attaches to the plain form or noun＋の to end a sentence, expressing one's own feeling, and can also convey a vague memory of the past; 「もしかしたら」 is an adverb expressing uncertain conjecture, so they differ in connection and cannot be directly swapped.
• 4.
  どうやらN2「どうやら」 draws an inference from signs with fairly high confidence, pairing with sentence-final 「らしい」 or 「ようだ」; 「もしかしたら」 expresses low confidence and pairs with 「かもしれない」. 「どうやら」 also has the meaning 'finally/somehow', so the two should not be interchanged.
• 5.
  はずだN4「はずだ」 is used at the end of the sentence to express a well-grounded, fairly confident inference ('should be'), and 「はずだった」 can express what was originally expected; 「もしかしたら」 is an adverb with low confidence that usually pairs with 「かもしれない」, so they cannot be swapped.
• 6.
  ないとも限らないN1「ないとも限らない」 attaches after a negative form at the end of the sentence, is rather formal, emphasizes that 'the possibility cannot be ruled out', and is often used to explain reasons for taking precautions; 「もしかしたら」 is an adverb placed before the sentence, usually pairs with 「かもしれない」, and is neutral in register.

かもしれない is the sentence-final expression of possibility that もしかしたら often introduces, and だろう expresses a more confident conjecture; ような気がする states a subjective feeling rather than an uncertain possibility. どうやら marks an inference drawn from evidence, はずだ a logical expectation held with confidence, and ないとも限らない (formal) means a possibility cannot be ruled out.