Why does everyone think weight limits is a good idea? That severely limits your storytelling capabilities. Would you really like to make it so Evan Bourne, Kofi Kingston, Cody Rhodes, etc are never able to win a world title because the Cruiserweight championship is the top they can get?
WWE is about storytelling, not the athleticism, which allows us to have someone like Big Show win the WWE title and then John Cena can. A smaller guy like Chris Jericho is acceptable as the WWE/WHC title holder and he's not 300 lbs. Anybody who is worthy of the main event in terms of entertainment is able to win the world title, while anybody who is not ready for that spot can win the midcard titles (IC/US). That's simple and effective. If you suddenly make it so only heavyweights can win the world titles, then we'll constantly be seeing Mark Henry vs Khali and you'd never see Jeff Hardy vs Edge.
So since the WWE is about telling a story and not the athleticism, the belts have to reflect that. One world title + one midcard title + one tag team title + one women's championship per brand (within reason). ECW doesn't have enough to support their solitary title, so they're struggling. Raw/ECW/Smackdown would have enough for ONE tag team division, not 2 or 3, so it'd make sense to unify those. For all intents and purposes, the women could have one division and wrestle on all three shows. And since you don't have enough for a solid women's division or a solid tag team division, an intergender tag team championship wouldn't work. If you increased the size of the rosters 3x more than what they are, yeah, it'd definitely work, but with the low # of talent they have, it's impossible to sustain that.
The only other type of belt that they could have that would serve any type of purpose is to bring back the Hardcore title or a variation of it. That gives you a different set of circumstances for a basis of feuds around the title and if booked correctly, it would help put some stars over. Imagine the storyline of having someone like a monster who, instead of just beating jobbers for 5 months in squash matches and then losing to the stars, is actually booked as the unbeatable Hardcore champion - the guy's allowed to be as ruthless as he wants to be, and even though everyone is allowed to use weapons, so is he, and you don't want to mess with him. Then, when you want to have him lose the title, you've got a perfect method of allowing him to lose but still keep his credibility, because its no-DQ.
Hardcore title is the only one that would make sense to add, and with the ECW title existing right now, there's no room for it. If they were to get rid of the ECW brand and ship the roster members off to Raw and Smackdown, merge the tag team titles, and add the Hardcore title as an inter-brand championship, it would work very well in my opinion for the people that were currently being neglected in the main event or midcard title scenes. Other than that, I can't think of any type of titles that would benefit the product.