I decided to stay with the crystal theme, and I think the interactive tutorial does a reasonable job. The last part of it is a sandbox so that you can practice all you want.
The next step is to bring in the uLink network engine and start producing some levels with online and "fake AI" multi-player support. Eventually I'd like to also add "boosters" that randomly appear in the paths. When hit by a domino, the player or team that belongs to the domino would get an offensive or defensive perk including: Reduced button cooldowns for yourself, or screen obfuscators for your enemy.
I'll make another entry once the multi-player part is done, hopefully in 2-3 weeks.
Does anyone have suggestions for the music and sound effects genres? I'm thinking either techno or metal...something exciting to keep up with the pace.