I am sure many people could guess my answer...

Yes! It's Adjacent Attacks.
With Adjacent Attacks, you can only attack from the territories you owned at the begining at the turn. So the game resembles more like "real" war. IMHO, it involves a lot more of planning and strategy, because it's not only about building a huge army to storm the board with. It's about actually battling around all the board. Sometimes with several fronts, and sometimes leaving one of your fronts undefended to concentrate on the other front. Something impossible with Unlimited Atack, because if you leave one border open, then your opponents can enter to the very center of your empire.
No, don't look for Adjacent Attacks in the game settings for the New Game screen. it's something not implemented yet in the site. But a group of passionate members play Adjacent Attacks simulating it by gentleman's rule. I have hosted two and a half tournaments with this rule (the first was just co-hosting) and I am positively surprised because everybody playing follows the rules, and incidents are very few. However, I look forward to this suggestion being implemented in the site.
In fact, I was about to leave the site when the first Adjacent Attacks tournament started. It convinced me to stay around.
I know some people will disagree with me, but in my opinion, implementing AA would attract a section of the user base that right now is either not getting into CC or leaving after some time. It's something that is just not covered with current options. So it would be not just another variation, but a completely different (and in my opinion more enjoyable) style of game.