Well, I´m not a SRB2 developer, but there are still plenty of Sonic gimmicks that could be used for DCZ, just naming a few already existing ones in classic Sonic games:
* Seesaws are common in classic Sonic games, but they aren´t implemented in SRB2. Hardest part would be to make them look good in 3D, and how Sonic stands on it (as it´s technically a slope, although it wouldn´t need to be like this in SRB2 to work).
* Some kind of bumper. It would make Sonic constantly jump up below jumping max height.
* Some kind of platform with different core gameplay, like Marble Garden Zone on Sonic 3, screws in Metropolis from Sonic 2... Sonic stays on it and can move the platform while on it. Jumping makes you jump from it.
Some more platforming sections wouldn´t hurt too, there aren´t too much of them in SRB2 yet, and it would fit the city theme quite well. Players need some more platforming elements before getting to ERZ too. I don´t know, I believe thinking more gimmicks is easy (specially looking back at classic Sonic games) but implementing them is always hard. I personally was amazed about things like the chains in CEZ, I didn´t expect them on SRB2. Those are the little things that makes this fangame so special.
About Red Volcano, I personally think that both zones should be named the same. Beating RVZ1 to reach BMZ2 would be confusing. I love the idea of an ice/snow zone with some lava, if the interior parts of the levels keep the red theme it wouldn´t be confusing at all. Another way of doing it would be having some kind of different titlecard for RVZ2. You would start the level going up on the platform of Red Volcano 1, with the title card of Red Volcano Zone Act 2. Then you would get outside and the "Red Volcano" letters would switch to the "Blue Mountain" ones. That would be more of a surprise than a confusing change.
Just throwing some random thoughts at the topic.