This exercise also appears in English from Spanish https://forum.duolingo.com/comment/750411/Hay-otra-cosa-a-considerar
A post there says: "La construcción "a + infinitivo" (a considerar, en este caso) no es correcta en castellano. Se trata de un galicismo."
The use of 'a' is required in a number of situations see e.g. https://www.thoughtco.com/using-the-preposition-a-3079307 But this is not one of them. It's not the first time I've encountered an unnecessary 'a' in this set of exercises. It seems to be a Latin American usage. Omitting the 'a' should not be rejected. Educate Duo by reporting.
"Something else" should be correct. http://www.spanishdict.com/translate/something%20else
Did you report it?
Why the "a"? Literally, Wouldn't this be This is another thing to to consider. I thought the "to consider" was built into the infinitive. It is perhaps something to do with thinking? An internal reflection?
"a considerar" is a prepositional phrase, where "a" is the preposition and "considerar" is the object of the preposition. While the infinitive "considerar" is usually translated "to consider", it's not quite the same as the English form. Another way to say it, a bit awkwardly, is "for considering".