I would definitely invest in:
Aqua Painter
Whisper White/Very Vanilla Cardstock (to watercolor on)
Black Stazon and Stazon Cleaner...LOVE the cleaner!!!
Versamark Pad
A few Stampin' Write Markers - to "isolate" areas on a stamp, or color in small areas...to start off with, just pick colors that coordinate with cardstock you already have, and possibly from the rich regals or soft subtles, so you'll have some form of "ink" from every color family? Also, base your colors on the stamp sets you have so far...if you have floral sets, make sure to buy a green marker for leaves/stems, etc.
Stampin' dimensionals - cheap and they last forever!
"vintage" brads (these are the assorted metal colored ones) - an inexpensive way to add simple but classy embellishments that match everything!
Snail Adhesive and Tombow glue
White grosgrain ribbon, and ink refills to dye it - this way you're not stuck with only 1 color of ribbon. Anytime you need a new color, buy the cheap ink refill and you're all set!
At least 1 background stamp - either "En Francais", "Sanded", or "Friend by Definition", depending on your style.
Paper Trimmer - could not live without it!!!
Hope this helps, although it'll hurt the bank account I'm sure!
Have fun picking out all your stuff!