How much is too much

Refine the basics first?
Then consider this question again. But with the intention of an expansion set to the game. That is, if that is possible.

I'm glad you're approaching this cautiously - it can be easy to get carried away with new ideas and new layers, and make a convoluted mess of what once was a great game.

Then the exact opposite can be true - one extra detail or a change in direction could make all the difference!

I'm actually at the same point as you are with my game. I've even tried out several variations that I liked in theory, only to find that they didn't work as well as I'd hoped, so I went back to my original rules. Most of the time. There were some changes I kept, but the biggest changes that I tried just left too many things to adjust in the rest of the game.

My advice, which may not be good at all - take it for what you will - is to put together the stuff you need for your new layer of complexity and play-test it. It's the only way you're going to know for sure, but more importantly, it's the only way you'll be able to give up on whichever version ends up being worse. Without trying, you're always going to wonder if your game could've been better in some way.

"My game is simple and straightforward" and "the rule book is very long" are contradictory statements. Either you just did a bad job explaining the rules, which gets unlikelier the longer the book is, or you are fooling yourself.

Game complexity needs to be an emergent property as much as possible. Rules, especially those handling special cases, need to be as limited as possible.

If your rulebook is so long you feel the need to mention it while defending the simplicity of your game, then you probably need to look for existing parts of the game you can thin out and simplify, before adding new material.

Have you thought about creating a play-aid for your players? What I mean is something like a card or small sheet of paper that indicates "On Your Turn" or "In One Round" or something like that. This can distill the game to its core actions, and allow players to refer to the rulebook occasionally instead of frequently.

It can also clue you in on areas of your rulebook that need clarification. The result is that you can spend less space on the stuff that's easy for a player to pick up, and more time and space on the edge cases or instances of complex rules/situations. This will naturally reduce the volume of your rulebook, and in a worthwhile, logical way.

Creating a play-aid is a useful design exercise, too. Your avoidance of "writing punchy rules" hints perhaps at a skill you don't think you possess. Give it a try and you might surprise yourself. :)

There's no real substitute for taking notes as you play, then transferring/backing them up on a computer somewhere. If you're hoping to progress with this project, I'd suggest you work this process into a habit. Secondly, it might seem like an immense chore each time you consider editing a 20+ page rulebook. You may find that, once your rules are a little less thick, you won't resist the urge to dig in and change a few things here and there.

Practically speaking, after your rulebook has been trimmed down it's likely you won't have to backtrack and/or check several different sections in your rulebook to make sure you've caught everything.

You can have a SUPER easy game - that can be taught in UNDER 5 minutes... And yet it takes a 20 page rulebook to "explain" HOW to play... The game can be so simple that a nine (9) year old understands the basics (or "core") of how to play the game...

Which is cool - because most kids want to PLAY "Games suited for an OLDER audience"... Kids like to play on the their bicycles - but secretly they all would love to ride on a dirt bike (that sort of mindset)...

So if your game is a bit more mature - but allows kids to get in on the action ... Great! But if the game rules can be read by a Parent and then explained to their children... If the game can be TAUGHT in that manner, I'd say you've got a pretty good game to share with kids, Parents and Gamers alike.

You mentioned about the possibility of "Expansion" and not "overly complicating" your "core" game. My advice - simplify the "core" and add "Expansions" to add more complexity to the game... Or perhaps more variety and maybe cards that go hand-in-hand.

Personally I think designing FOR "Expansion" is a smart way to go. It's why I am the creator and pilot for the XTG3 initiative: http://www.xtg3.org

I think games designed for "Expansion" should stand out from their game peers because the designer had the Vision to decide he wanted to have his game grow. Not an after effect of great sales - the idea that a "core" can be FUN and still allow for the possibility of new additions to "raise the bar" on the strategy, challenges and obstacles that must be overcome!

Cheers!

It might also help to have the game playtested without these additional rules, and then ask your testers if they feel something might be missing from the experience and what that might be.

Maybe some are disappointed about some parts of the game not having more to it, or feeling it does not make sense that they can't do this or that.

Through that it might help you find what may be missing.

Measure the time you need to explain the rules to beginners.

That should tell you how complex your game is.

If it takes more than 30minutes to teach the rules.. its complex... if it takes 45 minutes to teach the rules... its too complex.

Also measure the time you need for each player to take a turn.

if a turn takes 3-5 minutes, its complex... if it takes more than 5 minutes.. its too complex..

You get what I mean? the numbers are just from games that I have played. You might feel different