Bronto is cool, he's got a body and all, but he's more mana and not instant. Definately an option.
Activating Brontodon is instant speed, only costs 1 to activate too. Unless you need the anti-graveyard aspect I think he's clearly better than Return to Nature.
Activating at instant speed is very different from being an instant, given that the body can be removed before exposing your key enchantment.
And the graveyard interaction is extremely relevant in numerous matches (Phoenix, Golgari Adventures, Feather). I certainly wouldn't discount it. It's a huge bonus to have on an already relevant sideboard card.
I suppose Brontodon can be removed before exposing the enchantment, but 4 toughness is reasonably difficult to remove (Deafening Clarion, Skewer the Critics, Stomp all don't do it), it provides a clock, at 1 mana to activate it's cheaper than Return to Nature, and it's immune to discard. Of course there are pros and cons to both cards but I'm favoring Brontodon right now.
Is Feather still around? I've not seen it at all. Phoenix is rare too. The most common deck I'm playing against is Jeskai Fires. I have been playing some of that myself, it's good, but the mirror seems like a coin flip since whoever gets to play Fires first pretty much wins, and there aren't that many ways to interact with the powerful enchantment (it costs 3 to counter with Mystical Dispute, too). Aether Gust and Teferi could bounce it, but those are barely effective. Maybe the list I'm playing needs Disenchants or something in the sideboard.