KIA and maybe Reversed Najdorf starting with English 1. c4 or even Anderssen's 1. a3. 
Aggressive opening for white after 1.e4 e5?
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Possibly of interest:
My First Chess Opening Repertoire for White by Vincent Moret
https://www.newinchess.com/media/wysiwyg/product_pdf/9033.pdf
https://www.chess.com/article/view/how-to-understand-openings
A SIMPLE CHESS OPENING REPERTOIRE FOR WHITE by Sam Collins
http://www.gambitbooks.com/pdfs/A_Simple_Chess_Opening_Repertoire_for_White.pdf
http://www.jeremysilman.com/shop/pc/A-Simple-Chess-Opening-Repertoire-for-White-76p3916.htm
Winning with the Slow (but Venomous!) Italian by Karsten Müller and Georgios Souleidis
https://www.newinchess.com/media/wysiwyg/product_pdf/9037.pdf
The Four Knights: Move by Move by Cyrus Lakdawala (2012)
https://web.archive.org/web/20140627040728/http://www.chesscafe.com/text/ebcafe06.pdf
https://web.archive.org/web/20140627104938/http://www.chesscafe.com/text/hansen159.pdf
Playing 1.e4 - Caro-Kann, 1...e5 and Minor Lines by John Shaw
http://www.qualitychess.co.uk/ebooks/Playing1e4CaroKannandothers-excerpt.pdf
A Chess Opening Repertoire for Blitz and Rapid by Evgeny Sveshnikov and Vladimir Sveshnikov
https://www.newinchess.com/media/wysiwyg/product_pdf/9020.pdf
The King’s Gambit by John Shaw
http://www.chessvibes.com/?q=review-the-king%E2%80%99s-gambit
https://www.newinchess.com/media/wysiwyg/product_pdf/7093.pdf
The Ruy Lopez: Move by Move by Neil McDonald (2011)
https://web.archive.org/web/20140627022042/http://www.chesscafe.com/text/hansen153.pdf
Wrong forum.
Thanks for the suggestions, I'll look into them 
EzioAuditore96 wrote:
I used to play the Ruy Lopez but I wasn't having much success with it. What kind of aggressive systems should be recommended for someone who also plays openings like the Najdorf and the King's Indian? I've been looking at things like the Scotch Gambit but not sure how sound it is.
Scotch Gambit is considered to be one of the most sound gambits there is. I mean, it's a bit better for black with perfect play, but as far as gambits are concerned it's very playable.
It's funny, because 1.e4 is supposed to be the move favoured by aggressive players, yet after 1...e5 the best opening is either the Ruy Lopez or the Italian, both of which generally create much more positional games. Sure, the Italian can be played more aggressively with an early c3 and d4, but the antidote with the quick d5 has been known for literally a century now.
Most of the truly aggressive approaches involve gambits, be it the Max Lange, the Evans, the various Two Knights lines, and not everyone is thrilled with sacing a pawn so early in the game.
If you want an unbalanced position with attacking chances, there's something to be said for the Center Game: 1.e4 e5. 2d4. You can play like the mainline with 4.Qe3 or like a Scandinavian with 4.Qa4. You will play Nc3, Bishop somewhere, 0-0-0 and try to attack on the Kingside. Granted, Black gets a perfectly equal game with his own chances, but he likely doesn't have much experience in these lines. If he's a Berlin or Petroff player, he might be in the exact type of game he has hoping to avoid. There's room for creativity on both sides and not much theory to learn. Might be worth a shot, at least as a surprise weapon.
I suppose it might somewhat defeat the purpose of avoiding theory, but on the off-chance that it is of interest, there are about 100 pages on the Center Game in Dangerous Weapons: 1.e4 e5 by Emms, Flear & Greet (2008).
https://web.archive.org/web/20140627091711/http://www.chesscafe.com/text/hansen109.pdf
The center game
h4_explosive wrote:
... Scotch Gambit is considered to be one of the most sound gambits there is. I mean, it's a bit better for black with perfect play, but as far as gambits are concerned it's very playable.
https://static1.squarespace.com/static/5856bd64ff7c50433c3803db/t/5935c001a5790a30526c3cc4/1496694786380/scotchgambitexcerpt.pdf
I used to play the Ruy Lopez but I wasn't having much success with it. What kind of aggressive systems should be recommended for someone who also plays openings like the Najdorf and the King's Indian? I've been looking at things like the Scotch Gambit but not sure how sound it is.