What I've done is plant an idea in your head.
What I've done is chosen the products of several investment companies with proven track records.
I was wondering why the bare infinitive sounds right in the first, but the past participle sounds right in the second.
In general, either the bare infinitive or the past participle is supposed to be possible in this kind of construction, with no special restrictions that I know of.
In English, [when a verb phrase is focalized], it can appear either as a bare infinitive or as a to-infinitive, as shown in (1c). However, when a progressive -ing form is used in the presuppositional clause, the focus verb has to be in -ing form as well (6a), and when a perfective -en form is used in the presuppositional clause, the focus verb can optionally be in -en form, as in (6b).
(6) a. What I'm doing is patting/*pat/*to pat the cat.
b. What I have done is taken/take/to take a taxi to school.
–"On the Syntactic and Semantic Properties of VP Foci in Pseudocleft Sentences in Japanese", Yuki Ishihara, p. 37
"What I've done is planted an idea in your head" doesn't sound wrong to me. "What I've done is choose the products..." also doesn't exactly sound wrong, although I do prefer "chosen". Maybe people who usually prefer past-participle forms in these contexts have more tolerance for "plant" than for "choose" because the phonological form of "plant" is similar to that of a past participle such as "burnt" or "meant".
While "What I've done is plant..." flows and sounds correct, it actually isn't. If you separated the contraction I've it becomes I have and you are left with:"What I have done is plant an idea in your head." Plant should become planted to ensure grammatical accuracy.
