The truth is, that all the openings have deep and complicated theory. Even the relatively "elder""older" openings such as the Evans gambitGambit, Italian gameGame or 2Three Knights Defense used to haveGame had their theory to be developed realreally deep in the 18th, 19th and early 20th centuries. But luckily there are openings that are relatively sound, but. But they are rare nowadays, so by learning the basics of such openings you can go well against the opponents who didn't learn these openingopenings.
What I use:
White
- Vienna Game
- Italian Game
- 2Three Knights DefenceGame 1 e4 e5 2 Nf3 Nc6 3 Nc3
The vast majority of your opponents after 1. e4 e5 would expect the Ruy Lopez, so many are not ready to oldfor the good old Italian or 2Three Knights. Of course, these openings are not as good for whiteWhite as the Ruy Lopez and Black if playsthey play correctly can have equal chances, but the point is that few people actually know how to play in these openings and I quite often managed to achieve superior positions against much stronger players than I am.
Also when playing 1. e4 you of course should be ready to face the Russian game [Petrov's Defence, 1 e4 e5 2 Nf3 Nf6].
Against the Sicilian I play 1. e4 c5 2. c3 [the Alapin Variation]. A sound and good variation that is not that complicated as the main lines of the Sicilian. Also it can often end up as a classical variationthe Advance Variation of the French defenseDefense (1. e4 e6 2. d4 d5 3. e5 c5). [1 e4 c5 2 c3 e6 3 d4 d5 4 e5 transposes to the position after 1 e4 e6 2 d4 d5 3 e5 c5 4 c3 in the French Defence, Advance Variation]
Against the French defenseDefense I play the just mentioned above classical variationAdvance Variation. This allows me to feel comfortable in both variations as they can be very much alike.
Against Karothe Caro-Kann I play 1. e4 c6 2. d4 d5 3. ed cdexd5 cxd5 4. c4. Panov's Attack. Gives whiteThis gives White a strong and active position and it is you and not your opponent who chooses the variation, so you don't have to study anything but this variation.
Black
Against 1. e4 I always play the Sicilian Defence, Sveshnikov variationVariation [1 e4 c5 2 Nf3 Nc6 3 d4 cxd4 4 Nxd4 Nf6 5 Nc3 e5], though it is far from being "not deep" :)
Against d4 I like using the Volga gambit [the Benko Gambit, you1 d4 Nf6 2 c4 c5 3 d5 b5]. You sacrifice a pawn but get 2 open lines for your rocksrooks and your queen. In my memory I cannot recall losing a single game in this variation except when I played against IMs and GMs.
Against c4 I play as was advised above the hedgehogHedgehog system. Which is also far from being "not deep" :)
So to make a conclusion, I would suggest you to choose variations that you can enforce (such as Panov's Attack in Karo Kannthe Caro-Kann, or the Vienna Game after 1.e4 e5). When doing so you limit the number of variations you should learn and you can concentrate on learning them deeply. This will give you a huge advantage over an unprepared opponent especially when an opening is full of tactical tricks and traps such as the Vienna Game, Sveshnikov variation or 2Three Knights DefenseGame.
Good luck!