I'm choosing 'i' because it's a high-frequency vowel in English that I haven't guessed yet. Vowels are important to identify the word structure, and 'i' commonly appears in words where 'e' is the second letter (like "decision", "feminine", "beside", etc.). Since I've already tried 'a', 'e', 'r', and 't', adding another vowel will help me narrow down the word pattern.