Very useful information I found on merging season.
If your idea is largely on-topic on existing site, then It is better to contribute existing site because bigger is better:
The Stack Overflow experience taught us one thing pretty unequivocally: bigger is better. We didn't make Java Overflow and .NET Overflow and Ruby Overflow, we made one big site for all programmers and told them to use tags and, lo and behold, it worked. Why?
Critical Mass. A wider site is more likely to attract enough people that questions actually stand a decent chance of getting answered.
Rich, interesting information. A broad site is more likely to attract people who want to learn something a little bit outside of the problem that they're having right this minute.
Easier to remember and share. Stack Overflow grew to be successful because programmers told each other, "try asking on Stack Overflow." If we had made 2500 different sites for every possible niche programming technology, nobody could have known about all of them.
So. We should just make one or two gigantic sites, right? OK, done.
[There are also some examples that explains what's the right size domain for a Stack Exchange site? interesting - check it out!]
We need some rules!
Here's the best we could come up with for deciding whether site X should be subsumed by site Y:
Almost all X questions are on-topic for site Y
If Y already exists, it already has a tag for X, and nobody is complaining
You're not creating such a big group that you don't have enough experts to answer all possible questions
There's a high probability that users of site Y would enjoy seeing the occasional question about X
Note: This rules are also shown on FAQ.
Right now, there are a ton of active proposals on Site 51 which are, in our opinion, too small to justify their own Stack Exchange sites.
That said, we're firmly committed to the ideal that the community itself has to make the ultimate decisions, so in the coming weeks, we'll be building mechanisms that make it possible to discuss and hash out possible proposal mergers. In any case, there's a huge amount of value to getting these small factions together
so they can join forces in setting up large, robust sites that genuinely make the Internet a better place.