Bad. We'd have PvP whores trying to find the largest populated unclaimed one and buy it, then say they can kill anyone who enters.
Also, systems are done in an RP context. Faction systems are NPC ones first, PC ones afterwards. No matter what a player faction wants in their systems, it has to still be in-RP.
Furthermore, we don't have anywhere near enough systems for personal players to own one.
Over the course of some months, build up an RP character's reputation with various factions around Sirius and community members. A reputation for being say, super rich. With the massive riches then they might be able to 'purchase' an un-used system somewhere with their laws enforced by hired thugs.
That's an in-RP and perfectly acceptable way of doing precisely what you want.
Oddly enough, I have similar plans:)