I don't think that estonteco is any more out there than many European languages' coinages for "future", e.g. Finnish tulevaisuus, French l'avenir, or Romanian v

I don't think that estonteco is any more out there than many European languages' coinages for "future", e.g. Finnish tulevaisuus, French l'avenir, or Romanian viitor which are all derivations of the verb "to come" and foreign learners don't really have any problem figuring out the meaning. In fact, now that I think about it, Zamenhof may have calqued the Esperanto word specifically on Russian будущее "that which is to be", i.e. " the future".