Disclaimer: I am not a lawyer & this is not legal advice.
The existence of clones doesn't make cloning legal. Whether or not some other IP holder choose to litigate their imitators has no bearing on whether or not the IP holder you might be infringing upon will choose to litigate you.
Since the question of how closely can a game legally resemble another has already been addressed, the remaining issue is whether or not having worked on the game grants you use of the IP.
Ownership of creative work depends on legal jurisdiction & any binding contracts that might be in place. under US copyright law, the designer automatically owns all rights to the work they do, with the following exceptions:
- When work is created by an employee in the course of employment, the employer owns the copyright.
- When work is created by an independent contractor and the independent contractor signs a written agreement stating that the work shall be "made for hire", the entity commissioning the work owns the copyright if the work it falls under certain categories. The categories in question include audiovisual works, which I believe would include games.
- If the creator has sold the entire copyright, the purchasing business or person becomes the copyright owner.
If you want legal advice as to which parts of this are applicable to your specific case, you should consult a lawyer versed in copyright, preferable one with experience in the games industry.