As far as first decks go, this is really solid. Defenders and hydras have a lot of synergy and mono colour keeps the mana simple.

There are a few options to play with- remembering that power gaming with a casual playgroup is usually a mistake. Aim to be fun for everyone.

Turn 1 plays are really important in modern: Portcullis Vine Cathedral Membrane or Jaddi Offshoot are all cute here.

There are a few infinite combos with mana+untap in defender, but it's usually not too fun.

In general terms, when you onlu have one deck try for card variety with a consistent core -12 to 16 4 ofs that do one thing and make the deck reliable (defender ramp) and some varied payoffs to keep it interesting. I like Mistcutter Hydra for haste, Squall Line to tie games and Uncage the Menagerie to fetch those low cmc hydras or backup ramp.