If you make a fully tricked-out Goblin deck that uses no Game Changers, it's probably not a Core deck despite technically meeting the deck-building rules
Personally, I hope that Wizards takes a less sensible approach here
Gyruda, Doom of Depths, Lorthos, the Tidemaker, Stormtide Leviathan unleash devastating sea creatures
For instance, [c]Phantasmal Twin[/c] sacrifices itself when its targeted
Theres not a lot of cards with activated abilities which cost six or more (and thus benefit from both Zirda and Dynaheirs effects simultaneously), but there ARE plenty of X-cost activated abilities which will do the job fine
I think Endless One could be fine in a deck that has a focus on the +1/+1 counters on your creatures with cards like Avatar of the Resolute, but as a big payoff spell, the other options are miles better