Game designs, etc. are covered by patent, not copyright law. That is how WotC crushed early games that copied their M:tG playstyle- like the complete ripoff Martial Arts CCG called Ultimate Combat (b.1995-d.1995)
Whether something is copyright infringement or not is, as has been pointed out, not something determined by a rigid rule. Instead, its a factual matter that gets tried before the Court.
Basically, if you use someone's copyrighted material without express permission (outside of the statutory loopholes of education, parody, etc.), you won't find out whether it is or not until after you've spent a lot of money on an attorney defending your claim...and in the meantime, there will be an injunction barring sales of your product until the court case is settled.
Under the Copyright Act, four factors are to be considered in order to determine whether a specific action is to be considered a "fair use": the
purpose and character of the use, including commercial vs nonprofit or educational purposes; the copyrighted work's
nature; the
amount and substantiality of the portion used in relation to the copyrighted work as a whole (did you use the best of the meat or just nibble around the edge?); and the effect of the use upon the copyrighted work's
potential market value.
You might want to check out this commentary/summary:
http://www.bitlaw.com/copyright/fair_use.html
IOW, either drop the copyrighted stuff or get permission. If your stuff is really good, either try to get them to publish it directly (admittedly a long shot) or try to get it published with an established game company (whose lawyers/negotiation teams could probably swing a D20 license). Publishing this solo is probably an exercise in heartache.
OTOH, if your stuff is REALLY good...the word of mouth your game develops might just reach the right ears. Try running it at cons, etc.- they may come to
you.