There are a few police mods out there, most well known being it LSPD First Response.
But, these come with quite a few issues, like being not-persistent, not save-able, ugly characters, half-working to not-working missions, bad key configurations, silly and unfinished police stations, the list goes on.
My request would be a police mod, which uses actually the player characters Franklin, Micheal or Trevor.
Fully integrated into GTA5, so it becomes save-able at any given time, optional good looking alternatives to the character, nice police cars and-or vehicle skins, actually working missions using the normal user configuration, decent worked out police stations, in short, an actual police mod.
I am fully aware this is going to be quite a challenge, but I for one am willing to help where-ever I can, and maybe others would love to help too...