Boydie
Most people start off by saying things like "learn to use the tools you have" - but of course there is a step before this to choose what to learn! (chicken and egg!!!)

I'm old school, in professional studios back in the day (and now) if you need to do something you just didn't run out and buy more stuff to do it, you bought something because it filled in a massive hole, or used something that worked until it wore out, or replaced it with something proven to be much better. So of course I think of it the other way around, if you learn all the tools inside out then you are going to know what are the issues are with it, what you are missing, or what could be improved, leading to a better purchase decision. With Komplete and Sonar you probably have almost everything you need. Your own opinion on what you want is the most important and that takes experience.
 
If you do it the other way (don't learn the tools, just buy loads of plugins) you just end up with a shed loads of plugins that do similar things. You probably think you know what you want (something shiny perhaps), but don't really. I speak from experience at my end, I got trapped in this thinking (clever marketing is hard to resist) but soon wriggled out of it. That may appeal to people who spend most of their time grabbing the quick pre-sets that included within multiple plugins so I guess from that angle it is valid but can be extremely expensive.
 
Cheers...

mixmkr
Honestly, I'd look at stuff that is in front of your computer...like mics, pres, better sound sources and techniques.  That is much more suited it sounds like, to "go to the next level" in my opinion.  Then think about better monitors, acoustic surroundings...  you get the picture.

 
Excellent, excellent advice. If you can't hear what you're mixing properly, you won't mix properly.

This may be an open door but if you're producing vocals I'd get the Melodyne upgrade to Editor before anything else. Celemony has been kind enough to offer it for half off to Sonar users upon request, putting it at 150 instead of 300.

Other than that, I would have to agree with many here and get specific upgrades based on what you feel is missing, rather than a suite. If you need "DIY mastering tools", Ozone is a prime candidate. For some specific utility plugins or character plugins, some of the Waves offerings could be good. As Craig says their multiband limiting is good too. I love my Fabfilters but they also duplicate a lot of functionality you already have, not to mention their prices per plug are about what you want to spend in total. There are also many single plugs I like from various manufacturers. E.g. Cytomic The Glue, ValhallaDSP's Valhalla Room and Valhalla Vintage Verb, Stillwell's The Rocket, etc.

All in all, I would say it is not unreasonable to ask for more information. If you wanted a specific vocoder, leslie, reverb, guitarsim, compressor, whatever, you'd get a lot more educated opinions weighing in.

mixmkr
Honestly, I'd look at stuff that is in front of your computer...like mics, pres, better sound sources and techniques.  That is much more suited it sounds like, to "go to the next level" in my opinion.  Then think about better monitors, acoustic surroundings...  you get the picture.

 

Anderton
Excellent, excellent advice. If you can't hear what you're mixing properly, you won't mix properly.

+1

Lots of good advice in this thread...Alex, I know exactly what you mean about using what you have until one day, you find a limitation that needs to be addressed.

Ah... sorry. Wasn't sure how far ahead you were. Honestly I try to be minimalistic (mostly because I'm cheap) but to me those Softube suites designed to work with the Prochannel always looked cool. Old school emus that apparently work well and the convenience of having them in the PC is definitely a bonus.
 
Other than that as I said I personally really want some Izotope/Ozone stuff. Nectar was on the list but Melodyne seems to fill that void. RX would be super useful and all their fancy analyzers and corrections tools... well, just looks like good stuff.
 
Have you looked at the Melda freebie suite? Maybe there are some flavors in there you'd like (they seem quite popular) and they are free.

Thank you all sooooooo much for your considered responses and excellent advice
 
I jumped on the Melodyne upgrade straight away
 
This thread has really prompted me to try and identify any "missing" elements so I have put my money where my mouth is and posted 2 mixes:
 
http://forum.cakewalk.com/Some-Mixes-To-Help-Identify-Production-Mixing-Mastering-Improvements-m3103353.aspx#3103353 
 
If anyone gets a chance I would really appreciate any feedback on whether you think different plugins (especially mastering) would improve the productions and help "take them to the next level"

Its great music! I like it a lot. Both of them. 
 
My only comment on it is it may be a little too much bass. Then I was not listening on great speakers. 

I think I have the Waves Gold bundle - if that's the one I have then it has a ton of excellent plugs that I use all the time. The EQ's and compressors you get with it are excellent - VEQ is a really sweet EQ for making boosts and sounds stunning on vocals. Then you get the Renaissance EQ and compressor, both of which are superb and well used by pros (I see Dave Pensado use them all the time). There's also some fantastic vintage compressors and the wonderful H-Delay, my go to for delay almost every time. Then there's their amazing stereo widening and other spacial tools, Maxx Bass, a bunch of great modulation tools, a couple of excellent reverbs, some great guitar amps and stomp pedals, and a whole host of analytical tools. Plus loads more I haven't mentioned.

It cost me $300 at the time and when you look at the full price of these plugs it was a no brainer. I did have buyer's remorse for a couple of weeks, thinking I had succumbed to a bad case of GAS, but in hindsight I reach for these plugins every day because they are such top notch.

Then again, I could probably say the same for Melodyne Editor...

both mixes sound rolled of sharply at 10K to my ears.  that's your loss of sparkle.  stereo width seems ok.
 
as a tangent...on "shut the door", the opening guitars are VERY noticeably lowered after the vocals come in.  ...and the overall sound gets that "blanket" thrown over it. First a mix issue TO ME, then the EQ issue.  I didn't really listen much into the song, as you can quickly get an idea of the mix quality.

What are listening with on playback?  Something that is exaggerating your high end?
 
New plugs aren't the solution really at this point imo.   You need to address the lack of high end, that you're not hearing/recording somewhere along the line.  Same with some mix situations as well.