Waves Horizon question

None.
If you're covered you'll get any updated versions, fixes or plugins added to your bundle.
If not, you don't.

Agree, since they're always on sale now I doubt they'll add new plugins to those bundles like they did in the past.

My perception is that when a plug-in is performing poorly in sales, it is probably marked as "bundle-ready". For example, the V-Series plug-ins sold well when first introduced. Later they were added to the Studio Classics Collection, which would make selling the API and SSL bundles a little more appealing if the V-Series plugs were tossed in. As sales of that bundle cooled off, the V-Series was added to the Gold bundle after having been added to higher-priced bundles. It was one of the reasons I upgraded to Gold from Silver.
 
I don't believe even the endorser-named plug-ins/bundles are exempt from other bundles if the plug-in(s) aren't performing at a consistent pace. If they could be ready for deletion, placing them into a bundle increases the value of the bundle and keeps the plug-in(s) in use, perhaps. The only bundle/plug-in(s) still exempt from the Mercury bundle since over 3 years ago is the SSL4K bundle, which, in my opinion is worth every cent. It's always in high demand, apparently.
 
Mercury would be the first to gain new plug-ins, in hopes of attracting users to upgrade to Mercury. As Mercury increases in number of plug-ins, further separating the value of lesser bundles, those bundles will acquire the "left-overs" from Mercury. Although the Mercury bundle serves as the best value, doesn't mean the lesser bundles are worthless, or that Waves doesn't want to sell them. Waves sells them in hopes of getting you to use them, rely on them, then eventually go for Mercury. 
 
As for WUP, as the sales of plug-ins and bundles cool off awaiting newer ideas to come along, something still has to pay for support and updating the plugins to say, 64-bit and AAX (I openly ostracized Avid for AAX! It's a good idea---for them). It also helps encourage users to stay with later plug-ins, and wipe their hands of older versions. Lastly, as in my case, Waves was willing to work with me after-hours, late at night on an issue that was crashing Sonar X1/X2. They knew Sonar well enough to know how to fix my problem and test it. Paying for WUP allows Waves to support not only Waves plug-ins, but also Sonar's use of those plug-ins, not that Cakewalk can't support the same--just only during business hours.
 
TIP: When you wish to renew your WUP, Waves will entice you to renew WUP for all your plug-ins, or as few as you want, even just one. However, if you wait until your oldest (earliest expiring) WUP-supported plug-in is just over 30 days before expiration, Waves gives you a lower $ cap cost ($240 vs. $300 max) for that, BUT--if you renew ALL of your Waves plugins at once, Waves will (at the time of this post) give you a FULL year past your most recent plug-in's support expiration date. In my case, I gain over 1.5 years WUP support for one year's cost!
 
For example, say you buy a plugin with included WUP on January 1, 2012. Then you purchase another plug-in on Dec 15, 2012, which also includes WUP. If you renew your entire WUP for both plugins on November 30, 2012, not only will your Dec 15, 2012 plug-in be nenewed until November 29, 2013, but your January 1, 2012 plug-in will also be extended until November 29, 2013, giving you almost 2 full years of coverage. That may not seem too beneficial for just two plug-ins, but when you renew 30 days before the first plug-in expires, your cap is $240 vs $300. If you own 100 Waves plug-ins (or Mercury + SSL), this is a bargain!