Sorry, there was a lot to cover, so I did my best to address as much as I could

I didn't used to be a fan of chrome, but after seeing a dark blue lexus, which comes w/ chrome, I thought it looked much more classy than the Blue & Black scheme I would typically go for. So, the Rumion Rear License Garnish is on my list, but not at the top...
Can't view link here, but I think I know what you're talking about - pretty cool, but it would have low priority for me...
I don't think it's worth the money... there are other hanger/hooks that can be found for much less & perform the same function.
See my comments above about the chrome - same applies here, as long as they're not the cheapo covers, but the real deal
Window tint (Scion Dealer) $200ish
I don't know about your area, but you can probably get it done for much less. Make sure it's a good brand and it's guaranteed.
Stock mud guards $65.00 (Ebay)
Color matched? Good to go... If the price were better, they'd be higher on my list
TRD axel back exhaust $350ish?
I would consider that route, but I had a Magnaflow on my tC that I loved. I'd go w/ that again if I upgrade.
Black Neoprene seat covers $400 (Scion Dealer)
Something I hadn't considered. I'd like to see them in person though.
Black Dash cover less then $100 (Scion Dealer)
Not a fan... and I wouldn't expect our dashes to need the protection.
Something I hadn't considered. I'd like to see them in person though.
Something I hadn't considered. I'd like to see them in person though.
Got 'em. Like 'em a lot

The "deluxe silver" roof rack. For my boards ect. I called there company they said it should fit my car and hold over 200lbs. Cheep to I think it was like $160 w/ s/h. Customer service was very nice. I still need to do some more checking into this though.
http://www.perrycraft.com/roof_racks/Aventura.htmGood find! I like that much more than the other types
Rims (undecided)
Same here, but if I had the funds, it would probably be decided

Have it... it was okay, now... eh. See the post you linked to for my update.
Sound system (undecided)
Too many options/opinions. I'm over the big deafening sound & will eventually get a nice 10", amps, replace speakers & maybe headunit/nav.
Shark fin antenna (undecided)
Have it. I like the look...

LED taillights (undecided)
I like 'em. It's hard for me to justify almost $300 for those. I don't like any of the other options available though.
Chrome grill (style undecided)
I have the black Grillcraft & am now wondering if I should have gotten the silver to better match any chrome I might get... IMO, the stock grilles must be replaced and Grillcraft or billet are the best options at this time.
Newscionxb.com vinyl sticker
Is there any question? A must have!