"They'll" is a contraction of the words "they" and "will". As such, some synonyms for "they'll" could be various combinations of other pronouns and the verb "will". For example, "he'll", "she'll", and "it'll" all indicate future action by a singular subject. Likewise, "we'll", "you'll", and "they will" signify future action by plural subjects. Depending on the context, other synonyms for "they'll" might include "they are going to", "they intend to", or "they plan to". Ultimately, the choice of synonym depends on the specific language being used and the desired tone and meaning.