- Have a regular release cycle,
- Officially support OS X and Microsoft Windows,
- Not be useable by Maple/Mathematica/Magma, i.e., change the license of the combined work from LGPL to GPL,
- Be very friendly to new developers and their ideas for patches,
- Have a referee system, since technical knowledge about the inner workings of GMP is quite difficult,
- Everything would done in public, with public mailing lists, code repositories, bug trackers, etc,
- And be binary compatible with the official GMP for as long as possible.
The fork might be called MPIR: Multi-Precision Integers and Rationals, which is a name that is similar to the name of MPFR, the floating point library that builds on top of GMP.
Let's hope that forking GMP is not necessary!