Haven't really discussed much in the way of Merch - but you're right.
I'm sick of the shops within the parks that sell surf-wear brands without any tie-in (rip curl \ billabong \ roxy etc) - like at WnW I get it - if you visit and you don't already have something to swim in, that'd be nice, but not as much of an issue at the other parks.
One of my favourite souvenirs to pick up at a park is a polo shirt. I live in polo shirts, so i'm always looking to add to the variety. The parks always used to have quite unique offerings in this area, but now there aren't many options - and the option they do have is the same shirt with a different logo (eg: sea world sells it with a dark shark sillouette, movie world sells the same shirt with a black and gold movie world logo). On my last visit there was maybe one other option - same shirt style, but in a different colour, with the sea world logo in white, but no other options.
There's always plenty of t-shirts - and sure, t-shirts are easy to spit out mass produced vinyl printed imagery for any random event you decide to throw on - but put a little flavour into something a little classier. Make them a limited batch for each event - even if they sell out early - i guarantee there are people out there who want more than just another t-shirt (plus the collar is more sun safe too!)