"I'm thinking of things beginning with the letter . ." I

Igloo
  Imbecile
   Ink
    Irritate
      Island
       Imp
          Icky
            Isabella
               Irrational
                 Inland
               Ian
            Iceland
          Idyllic
        Ireland
  Important
Impractical
Ishmael
Imperial
  Interested
    Immaterial
      Imelda
        Instinct
         Ignorant
          Inherited
           Illustrated
             Immaculate
               Innovative
                 Iquique
                   Irregular
                     Istanbul
                       Izzy
                         Illegitimate
                            Ice
                          Imperfect
                        Ingredients
                      Instant
                    Institutionalized
                 Ignoramus
               Inspirational
            Intuitive
         Infinite
      Immeasurable