Anderton
SPlat still includes REmatrix Solo.

Great.  Then I'd say there is nothing really essential that isn't well covered by SPlat.  I am enjoying using Izotope Neutron, which covers a lot of territory occupied by the various SPlat plug-ins.  I find the integrated Neotron environment a little easier than using a bunch of separate plug-ins, but I tend to stick pretty close to the the presets.  And I certainly would not call Neutron essential.
 
If one is dealing with live recordings, then I would definitely consider Izotope RX5 to be essential.

I have said this on many occasions to members asking the same questions.  Sonar Platinum has everything you need for making quality recordings. Really, you don't need anything else except for a good audio interface a pair of near field monitors and the time to learn them fully. Here CW has a over 2000 page manual with all sorts of chapters dealing with subjects like digital audio and MIDI. It has tutorials that take you through applying these things in Sonar. Plus you have us. This forum. It prides itself in offering help for anything Sonar related. Get stuck ask here and I'll bet members will be falling over themselves giving great answers. A member leaves out something not to worry another will add the missing information.  Also you have years of knowledge accumulated over the years that only a forum with a long history can have. Further,  you want to watch a video or two on a specific topic check out the video archives here.  http://www.cakewalk.com/CakeTV/SONAR-University
 
If you love music and want to learn how to record it well you have come to the right place. 

John
Here CW has a over 2000 page manual with all sorts of chapters dealing with subjects like digital audio and MIDI. It has tutorials that take you through applying these things in Sonar.

That's too hard.  Isn't there some pill I can take to become a better engineer? 

Yes. Current version of Cakewalk Sonar Platinum does come with everything you need. I got rid of everything else. I bought all Prochannel plugins cakewalk has and I am so A-ok. Because now I use all the knobs there are and I truely are imimate with all the features. Gives me a comfy feel. And my remote controller works great with the system!