i would rather play black marsh more than anything...but this is a painful truth. swarms of blood flies, floods and even the very nature of a marsh would be next to impossible for bethesda to do atm (did you know cyrodill is actually, for the most part, a large forrest and not the endless plains oblivion made you play in?? i'd hate to see what blackmarsh turns into)
Actually, only half of Cyrodiil was described as tropical rain forest, in the Nibenay Basin around the Imperial City. The Colovian Highlands were listed as low hills and temperate forests, pretty much like what we got for nearly the entire province aside from the Jerall Mountains. In game, there was little difference between them (I won't claim "NO difference", but not MUCH of a difference), and you couldn't even buy a Colovian Fur Helm in Colovia, although they practically fell from the skies in Morrowind (you'd understand the reference if you played MW). There was no evidence in the game of the competitive hostility between the two regions, and not even a mention of them even being seperate regions. Bizarrely, there may have been hardly any more lore about Cyrodiil in Cyrodiil than there was in Morrowind.
Incidentally, the imperial Island was supposed to be nearly 100 miles long, most of it city, which would have made for a complete (and more believable) game without ever venturing off the island into the rest fo the province, as was done with the island of Vvardenfell in Morrowind province. Daggerfall attempted to represent the entire province in a realistic scale, which was impossible to do with any sort of detail. Morrowind limited the game to a sizable quarantined island in the midst of the province, making the size "scaled down, but somewhat believable". Oblivion attempted to fit the entire even larger province of Cyrodiil into a space only marginally larger than what it used for the island of Vvardenfell, making it feel absurd to cross nearly a thousand mile wide province in a few minutes of playing time, and seeing the White Gold Tower clearly from cities supposedly a few hundred miles distant. The in-game claim was that you could see the Tower from anywhere in the vast city, NOT the entire province. Skyrim stuck with the "cram the whole province into a shoebox" approach, leaving us with a few villages scattered across barely a county-sized patch of mountainous land and forests pretending to be an entire country in its own right.
Black Marsh itself may be somewhat of a technical challenge, but the animations for the various Kajiit sub-species (fully erect, digitagrade, and quadruped forms, plus a partially arboreal type) would probably be far more work. A significant part of Elsweyr is jungle, beside the large open deserts, so there's more terrain diversity than most Nords would comprehend (the ONLY race I haven't bothered to play at some point in the series).
I'm hoping for either Black Marsh or Elsweyr, but we're more likely to get Hammerfell or High Rock; another generic "man" province. Bethesda seems to be too afraid to offend someone by trying anything outside the box, thereby annoying and offending all of those who have been anxiously waiting in vain for anything worth waiting for. They're too busy trying to be the Wal-Mart of gaming, and catering more and more to a Wal-Mart customer base.
Akaryu claims that Skyrim is what the fans wanted, but previous polls here repeatedly showed Black Marsh as #1, and Skyrim and Elsweyr competing closely for #2, but we got Skyrim. AFTER it was announced, the bandwagon jumpers flocked to the Skyrim theme in droves, but now that it's over, Black Marsh is once again at the top of the polls, followed by Elsweyr.