to keep it emissions legal in most states, the motor has to be as old or newer than what you've got, so 3s-gte may be iffy.
I saw one for sale on e-bay, with tranny. keep in mind, you have a FWD car, most 3s-gte will be for RWD (MR-2) or all wheel drive (Celica all-trac)
an AWD celica would be cool, but you'd need the rear-end from the donor car, and you'd have to figure out how to mount it to your unibody (which means old rear suspension is gone). either that, or the rear-end from an AWD matrix may work easier.
MR-2 swap may be eaisier, but remember, the engine and tranny were mounted in the REAR of the MR2, which may open up a whole new host of problems. You may want to think about just building your current motor, or think up an alternate swap. look into Lexus models, the IS300 might be a suitable candidate, but You'll have to double-check clearance in the engine bay.
It's a LOT of work to get it to fit, but it's entirely possible. both sportshift transmissions from the is300 and celica GTS are almost identical, so it's entirely doable, just remember the IS300 is RWD, you'll probably have to do a rear end swap and some cutting of the firewall/fabrication to get the IS tranny to fit your car, the GTS sportshift is another story, since the inline 6 wont fit sideways under your Hood
without custom work. may want to measure to be sure, though.
I would swap in the 2ZZ-GE, best option would be the JDM 2ZZ, as it's rated for more HP and tourque, and has a higher redline. The transmission you have should work, since they are based on the same block, but the sportshift would be better and more reliable (that, or just swap in a manual tranny).
Compression could be lowered a little, and you could add a turbo, you could make plenty of HP, just don't forget to swap in some better axles to handle the added stress. also, don't forget, you'll need the ECU and wiring harness, as well as all the required sensors. doing the harness will be the biggest pain in the ass, BTW.
I like the rotary idea, it'd require some effort, but wouldn't be all that difficult, since the rotary is smaller, but the engine has to be from same manufacurer with current cars to be street legal.
In closing, I have three words for you: 3ZZ Stroker Kit!!!