Unfortunately, Nissa, Worldwaker is too slow for the pace of modern, if you still want a Nissa the girl for the job is Nissa Revane. The only card for Parallel Lives to be synergistic are Dwynen's Elite and Nath of the Gilt-Leaf. Damage generally by burn really hurts, I would recommend the use of Gilt-Leaf Palaces to negate this (would take out swamps). The greatest power of elves is the ability to ramp insanely fast. To better support this theme I would run the full 8x mana dorks (Elvish Mystic and Llanowar Elves. In addition to this the combo of Nettle Sentinel and Heritage Druid allow you to accelerate to huge amounts of mana. I would recommend taking out the Gnarlroot Trappers and Jarad, Golgari Lich Lord. Consequently, with all of this mana you need to do something with it. Generally, the best use is to deploy a finisher. The best ones are Ezuri, Renegade Leader (2-4) and Craterhoof Behemoth (1 copy). With elves you generally run out of cards. To mitigate this I would recommend running Lead the Stampede. If you would like to learn more I would recommend visiting the friendly subreddit of Elves to strengthen your deck and playing skills (It has helped me immensely even after playing this deck for a year).

Thanks for the suggestions Dursty. See, I never even knew Gilt-Leaf Palace was a card! I wanted to include Elvish Promenade as a more effective synergy with Parallel Lives on top of the two cards you mentioned as well as Imperious Perfect's ability, but that will be something to playtest. I actually have two Ezuri, Renegade Leaders that were in the deck until my most recent iteration, his abilities are certainly good but I wasn't finding him as useful, perhaps just because when I would draw him he'd either die before I'd have the mana for the +3/+3 ability, or I had so many buffed elves that at that point it didn't really matter. Definitely something I need to playtest more, though. And yes, I really should have Nissa Revane lol, I got lured in by Nissa, Worldwaker's abilities of untapping forests and turning them into 4/4 elementals with trample that are still lands :p I'll check out that subreddit too! Thanks!