I am on my second Tailwind, doing a ground up restoration of a 1958 W-8. My first was a W10. Both of my planes had/will have metal wings, so I will try and answer your questions. Check out groups.yahoo.com/group/Tailwindforum for a wealth of info.
1. I am building metal wings for my W-8. The W-10 and W-8 have the same wings, by the way. Consult the group for all differences between the 8 and the 10. There are a few, but basically they are the same plane. Callbie Wood sells metal wing plans for $75 or so. There are many drawing discrepancies between the old Wittman plans and the new Spruce plans. They are all spelled out in the group and also Jim Stanton has a construction manual he put together. It would be well worth the money to get. Also, if you want Jim Clements mods, they are also available. They are the ones that make the Tailwinds look like all the recent award winners do. Also well worth the money. My first Tailwind had a Riblett airfoil and 2 feet more wingspan. My climb was 2300-2400 FPM and it got off the ground quickly with 150hp. My W-8 (avatar) will have the Wittman airfoil and span as in plans with 100 hp.
2. Vortex generators would work just as on other planes. if anything it will help the initial slight "float" one feels right when a Tailwind takes off. A short field airplane a Tailwind is not overly suited for. Unimproved fields, definitely yes. Tailwinds are well known for their gentle stall characteristics.
3. Larger windows have been on several Tailwinds. Skip Stewart's W-10 (the airshow pilot) had doors that were mostly window. Jim Clement's mods provide for larger doors, among other things.
4. Folding wings could easily be adapted to a Tailwind. A lot of Nesmith Cougars have folding wings. That would be, in my biased opinion, the only thing I would borrow from Cougar plans. Most of the very nice Cougars have a lot of Tailwind in them. The Tailwind is just a much better airplane.
5. Nobody I know of has tried the Rotax. It might work in a W-8 airframe. A W-8 is essentially 6-7 inches shorter due to the larger tail surfaces required for the larger engines on the W-10. The plans show both. The best engine for the W-8 is the O-200 and the O-320 for the W-10. There is a recent one flying with a Mazda and a few have flown with Olds/Buicks. Only one owner I know of has been satisfied with this set-up. My O-200 freshly overhauled was $7500. The O-200 W-8 and the O-320 W-10 get roughly the same economy (a little over 30MPG) at the same speed and this has been proven time and again.
6. Check the safety record on the NTSB. About 2 1/4 pages from the beginning. Ground handling is essentially the same as a Citabria. Landing speed is about 5-10mph higher than a Citabria. A Citabria is an excellent transition airplane.
7. Big wing extensions over 4 ft should not be done due to the requirment of the wing geometry. If you want to redesign and test the wing go for it. My 26' wing span dramatically inmproved climb performance. Ground roll of 500 ft on pavement is possible with the Riblett airfoil, 26-7 ft of wingspan and probably an O-360 and VG's. Jim Clement's latest has an O-360. 10 pounds heavier and about $4000 more for the core with increased fuel burn if you open the throttle.
There just isn't anything better than a Wittman Tailwind. They are relatively easy to construct and it is definitely an inexpensive flyer for it's class. Hope this helps and good luck,