You've got a steeply
uphill battle ahead of you. Modern is a pretty brutal format, full of thousand-dollar decks that can win on turn 4, and you're handicapping yourself by sticking to a theme.
(Huh. Didn't expect it to autocard
uphill battle. Oh well.)
Anyway, you probably need more lands. 18 is not very many.
There are a lot of cards in your deck that just don't do all that much. Like
Sunbond, for example. It can easily be a wasted card if the creature you put it on gets sent on a
path to exile.
Vanguard of Brimaz,
Leonin Relic-Warder,
Savannah Lions, and
Ajani's Sunstriker are all worth a look. If you add green, add
Loam Lion,
Wild Nacatl,
Fleecemane Lion, and
Qasali Pridemage to that list.
If you add fetchlands, you might have some luck with
Steppe Lynx and
Scythe Leopard.
Brimaz is a very powerful creature. More copies might be a good idea even if he is legendary.
PS: What's up with the
Imposing Sovereign? Doesn't seem to fit.