If I created a seed accelerator program in New York City, can I call it icombinator or wcombinator?
What are the legal issues with copying the name of the famous seed accelerator program in California - YCombinator?
No, you can't do that.
If you did that, you would confuse the public. The public would think you were associated with YCombinator when in fact you are not. Further, you would be choosing that name deliberately to benefit from YCombinator's well known brand so you could even be liable for willful trademark infringement.
It wouldn't make much sense anyway.
"Y-combinator" is a clever name because of what it means: a function for enabling anonymous recursion in other functions.
"W-combinator" and "I-combinator" are just nonsense.