You don't lose unspent green mana as steps and phases end. Omnath, Locus of Mana gets +1/+1 for each unspent green mana you have.

This spell costs {X} less to cast, where X is the greatest power among creatures you control. Whenever one or more nontoken creatures you control die, create a green Fungus Dinosaur creature token with base power and toughness each equal to the total power of those creatures. {2}, {T}: Double target creature's power until end of turn.

Trample When Ghalta, Stampede Tyrant enters the battlefield, put any number of creature cards from your hand onto the battlefield.

Exile the top card of your library. If it's a permanent card, you may put it onto the battlefield. If you do, repeat this process.