Zib, that's fair enough - although Sci-Fi wasn't really a key background to this and, to be honest, sci-fi role-playing isn't exactly a concrete base from which to make grand suppositions! Those of us who actually work within large public service organizations (both uniformed and otherwise) have a pretty good idea how these things tend to work too!
To expand upon my concept for this organization, I also meant to add that the role of Warrant Officers (or equivalent "specialist" grades) within such a service would seem to be entirely reasonable: within an army, the core role of a soldier is to fight so a military policing role would rightly be seen as specialist role. Within a police service, however, the basic law-enforcement and investigation role is the core of the duties so therefore *not* really all that specialist! The truly specialist roles that require professional knowledge and skills above and beyond the basic core role might be other things like forensic investigation, vehicle examination, specialist fraud investigation (requiring accountancy skills), psychological profiling and so on. Now, these roles could be provided by non-uniformed civillian support staff but, depending upon the nature of the organisation (and I believe this could apply to our hypothetical international police service), I'm sure there is also room for specialist work like this within a sworn officer role - perhaps by some sort of direct entry professionally qualified route separate to the standard "police" training and career structure.
In the schemes below, I've outlined two parallel grade structures - one for the local regional / area / divisional structures (how big each of those sub-units would or could be is clearly open to debate! States...? Nations...? Continents...?) and another for the overall co-ordinating international structure. There's also the question of how such a service would work with local police services - would it replace (all or some) local services? Would it act like US State Police relative to local county/city police and/or sheriff departments? Would it have a role more like an over-arching federal service like the FBI / US Secret Service / US Marshal Service...? Perhaps, realistically, it would actually need to be some combination of all of the above?
For now, I've used plain silver on blue for the locality grades and gold with silver for the international grades. The international staff-grades and operational agents have gold-silver-gold cap straps that echo the lace of the rank stripes.
Note also that there are no international grades below deputy inspector for the international structure and also the use of warrant-type grades. This is because most of the international grades, even for operational agents, will be specialists of one sort or another. For example, think of the international-level A-1 grade agent relative to a local patrol officer (single silver chevron) as being the equivalent of something like an FBI special agent relative to a rural local county Sheriff's Deputy. Teams of agents would also incorporate the specialist roles described above, especially the forensic / financial / psychological areas.
Remember that, in this system, these grades also overlap the O-4 to O-6 grades; the way I see this is that the O-4 to O-6 staff "officer" grades with "stripes" are involved in international "policing" management and supervision, working to co-ordinate with local services whereas the grades I've described as A-1 to A-4 with WO-type chevrons (+/- bars) are members of operational investigation and/or intervention teams. The A-2 to A-4 grades are roughly equivalent to the staff O-4 to O-6 grades, with the A-4 team leaders reporting to O-7 Staff Superintendents within a specific international operations bureau. (Note that the pattern of bars beneath the wide chevron is slightly different to the standard WO pattern insignia used elsewhere).
The other thing to note is that I imagine many of these international-level staff would usually only wear uniform on formal occasions (especially the operational teams) but it's useful to see the grading system laid out by insignia.
Local grades and insignia:
[img]http://i655.photobucket.com/albums/uu277/bravo_whisky/WorldRanksPolice1.jpg[/img]
International grades and insignia:
[img]http://i655.photobucket.com/albums/uu277/bravo_whisky/WorldRanksPolice2.jpg[/img]
Uniforms (following the same basic pattern as all the others):
[img]http://i655.photobucket.com/albums/uu277/bravo_whisky/WorldRanksPolice3.jpg[/img]
A word or two about "detectives" - I view this as more a role than a grade in itself. At the local level, the basic "Detective" grade would be equivalent to the E-3 "Senior Constable / Officer" and, when in uniform (rarely, I guess), would wear that grade's insignia. A detective team would be led by a local-level specialist supervisor (senior sergeant II/III) who would equally wear their equivalent grade insignia when in uniform. The officers managing several such teams would likely be a Detective Captain, aided by a Detective Lieutenant with overall executive management of such groups of detectives by an allocated inspector and/or deputy inspector - they would wear plain clothes or uniform as required. The option also exists to have a "Senior Detective" at E-4 grade (Sergeant equivalent) who would retain sergeant-level seniority and authority and who may work as the supervisor of small local teams (in lieu of an E-5 specialist supervisor) or who may be an experienced team mamber on Area or Regional-level specialist teams such as major Homicide / Narcotics / Fraud / Counter-terrorism groups. Team leaders here could be at the E-7 Senior Sergeant (I) grade and overall management / leadership responsibility would likely rest with an allocated Inspector / Chief Inspector grade officer, assisted by more junior officers.