Thanks for the input. I guess I didn't realize that there are endless possibilities with 32 pieces on 64 squares... maybe I'll just try playing whatever responses that first come to mind, and take my chances with theory some other time.
Picking something for 1. e4 e5

By the way, how close are the similarities between the Ruy and playing c3 in one variation of the Sicilian Moscow? I've heard that they're somewhat close, and it would be great to have an opening overlap... I don't know for sure, though.

Seriously, they're all fine. And you left out the Vienna Game (2.Nc3), which has many elements in common with the King's Gambit.
In the Spanish, you can avoid an awful lot of theory with a timely d3 move e.g. 1.e4 e5 2,Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.d3, or 5.O-O Be7 6.d3. You still keep the essential "Spanish" nature of the game, but you avoid a lot of long, theoretical variations.
This is also true in the Italian.two-knights complex, where you can get a complex game wit d3, c3, and Nbd2. Or you can play down the more open lines if you like.
And judging from game 3 of the Kramnik-Aronian match, the tame old Four Knights can be pretty wild!
In all these openings, the type of game you get is pretty much up tp you
yup....actually there are quite a few playable offbeat options after 5...Nf6/5...Bc5 in the Spanish.6.Qe2 looks interesting too
@NachtWulf
My primary choice against the Sicilian is also Bb5+, in those cases where they interpose the knight 3...Nd7 it is indeed possible for White to get positions with pawn structures similar to closed Ruy Lopez. This requires Black to chase the bishop away (a6,b5) and then play ...e5.
Example:
Something like that. It has some similarities to closed Ruy Lopez positions, however 3...Nd7 seems to be the least popular reply to Bb5+, so I've hardly ever got a chance to test how well this variation works for White. The few times I've been here I opted for a setup which is somewhat similar to the Worral's Attack of the Ruy Lopez (Qe2, Rd1), but certain details make Black's life easier - compared to a Ruy Lopez. Especially their d7-knight is slightly better placed than on a5. Also compared to the Breyer Defence Black doesn't need to spend extra time to get their knight to d7 (via b8), because it is already there.
It should be mentioned that Black can very soon try to hinder White from getting their pawn center, e.g.:
Consider the Italian with c3/d3, aka "Pseudo Ruy Lopez".
Advantages:
- Plans are more important than theory. So there's a chance to learn something useful from playing it.
- Versatility -- you can play on the K wing, the Q wing, or in the centre.
- Also works against the Two Knights (and other deviations except for the Petroff).
Disadvantages:
- Definiteley slower than the KG.
- Requires quite some positional skills to be successful.
- Will take time to master -- no way to book yourself up quickly, lots of practice required.

What's the difference between the c3/d3 italian and the worrall? They seem nearly identical, but the worrall seems to waste a tempo moving the queen... did I miss something?
Firepower, why Scotch, and which variation?

what line did you use in the KG, cause the Bishop's Gambit will take most people out of theory immediately, and can give you a very quick attack if played correctly, plus it will catch most people by suprise when played.

NachtWulf, I have a few recommendations. I like the Italian Game, and the Fried Liver, but have a hard time fighting agains the Guoco Piano. Same with the Scotch. If they play 4....Bc5, I like the 5. Nb3 line, but I have a hard time fighting against 4...Nf6. Therefore, I recommend the Parham!!

I'm not the biggest fan of gambits, but seeing how nacht is coming off of a gambit, he might like that.
What's the difference between the c3/d3 italian and the worrall? They seem nearly identical, but the worrall seems to waste a tempo moving the queen... did I miss something?
[...]
In the Ruy Lopez, black has plenty of alternatives at moves 3 and 4 you have to cope with. So you might not get a Worrall. The Italian is a one-fits-all.
In the Worrall, black has IMHO better options for counterplay (in particular b5, Bc5, d5 and the Qe2 is a bit misplaced).

I don't think the Worall attack is that bad....it's been played by some strong players so can't be that bad
I don't think the Worall attack is that bad....it's been played by some strong players so can't be that bad
Well, it's certainly not bad. But note that in the quoted game, Anand played Qe2 only after 5.O-O Be7, so black can't go for the Bc5/d5 setup.
If you recommend this move order, you also have to prepare for the Open Ruy Lopez, the Arkhangelsk, and that fashionable b5/Bc5 stuff. Life's a bit short for us amateurs ...

In Dangerous Weapons 1. e4 e5 they recommend the Center Game. It looks playable & obscure & avoids the Petrov & long dragged out theoretical stuff.
1. e4 e5 2. d4 exd4 3. Qxd4

I've decided to play the c3/d3 Italian. Granted, it's completely new to me so I'm still very unused to it, but I like the way the resulting play "feels". Are there any books on it?
I've 'experimented' with the KG for quite awhile (as in, I use it as my one and only response against 1. ...e5) but frankly, it seems much better for blitz than as a permanent option OTB. The Spanish is neat, but seems quite theory intense in terms of having to know multiple mainline variations, as opposed to only sidelines of a single variation. (In other words, I'd like more 'choice' than my opponent when it comes to decisions that occur in the opening.)
At this point, I'd like to consider switching away from the king's gambit as my main opening against 1. ...e5 but I've no idea what to switch to. Correct me if I'm wrong, but my choices are the Spanish, Italian, Scotch, and Three/Four Knights. Frankly, I've too little knowledge of each to judge them for what they are, much less the main variations of each. I'm looking for something that emphasizes middlegame ability as opposed to endgame knowledge (the Sicilian might be analogous to the first, while the Caro-Kann or Spanish Exchange would be analogous to the second).
So... suggestions?