For future fusion, when 2 turns later if there is no CTOD, FF stays on field and nothing happens cause nothing to summon.
For overload fusion, difference cause a fusion card does not target. So there is NO declaring. But of course, he must have something else legal to call lah, example: gatling dragon, etc.
For fraken, its just priority to pay 5k to activate effect, it does not declare which it is gonna summon. Therefore, if he has fusion left, he can still summon it out. Same goes for illusionist master, etc