Working on the concept that the WWE will have a network within the next year and will add a couple shows to provide enough new programming, here are my thoughts about title structure.
To do the whole thing on weight classes would mean a complete restructuring of the entire WWE landscape. It would mean adding a half-dozen weight classes at minimum and would result in dividing the roster too much, limiting who could compete with each other.
Alternatively, having just a Heavyweight, Cruiserweight and maybe one or two other weight class titles, even with Tag and Diva titles, would be too few. Or more to the point, too much of the roster would be competing over the same title.
With the current WWE roster, you could have a U.S., European, and Mexican champion, a North American champion and a World champion. That seems like enough, but the North American champion would always be either the U.S. or Mexican Champion as well (unless he was one of the two or three Canadians).
Likewise, the World Champion, if he came from North America or Europe, would also be holding one or more of those titles. So when John Cena (or any other U.S. citizen) carried the World title, he'd also have the North American and U.S. belts as well.
Even if you didn't structure things that way, with only six (or fewer) men competing for the Mexican or European titles, it would get farcical. Sure, fewer men than that are currently after the U.S. and Intercontinental titles, but the verisimilitude is that there are a bunch of guys waiting in the wings for their shots.
And I don't see Vince cooperating with any foreign talent or companies (if there are good ones available) to occasionally lose belts overseas (even though they shared belts with NJPW at one time). First of all, WWE is supposed to be the global entity, and second, I just don't see him working well with others.
There has been no concrete plan to make the titles in WWE make sense; really, it's just grown organically, and titles have been added and dropped periodically. Tradition is about all we have to go on besides innovation.
However, I think it's clear that if WWE's network gets legs and there are several shows per week that each show needs its own title (or two). That way, the top two or three healthy performers in each show can be involved in chasing its title, with another few competitors just outside the picture, trying to get in
Titles will be able to be contested by any eligible person, no matter what show they tend to stay on (Raw, in particular, will collect talent from all the shows), but whoever is champion will appear on that show during their reign, even if they are contracted from another show.
Of course, certain titles (like the tag belts) will move from show to show.
In time, if there is enough talent and all the shows are succeeding, then things like Women's or Cruiserweight Tag belts could be added.
Also, replace the spinner belt, the tag team pennies and the Diva monstrosity.