Thanks for your reply (and also for your email). A few answers below:
1. The MEGA65 supports almost all C65 modes and features, including the bitplanes, although in my view, the C65's bitplanes are too limited to be really useful for anything, primarily because scrolling them requires moving a lot of memory around.
2. The MEGA65 supports new enhanced text modes, that includes both 16 colour 16x8 and 256 colour 8x8 character tiles. For both of those which require 64 bytes per character, you can have up to 2,048 characters, so it is quite easy to make large scrolling playfields this way. This will be great for platformers and similar games, and also helps save a LOT of RAM, when you tend to have large empty or repetitive parts of the screen. You can even use the hardware character kerning feature to allow characters to not be forced to the 8 pixel horizontal columns, giving some extra flexibility.
3. There is a hardware 25x18 bit multiplier that gives a 48 bit output. This takes only a few cycles. Basically you don't have to wait at 40MHz, because it is finished before you can read the first result byte. There will also be a "hardware spreadsheet" that can be used to implement simple equations, without having to copy operands around the place, making things potentially much faster -- faster even than many 32 bit processors can calculate such equations, due to the removal of the need to shuffle values around the place.
4. There is a C64 mode, and a C65 mode, as well as the new MEGA65 features.
5. Yes, a library of routines for doing interesting things on the MEGA65 and/or C65 would be very interesting and useful. Bonus points if you make the routines with CC65 wrappers, so that the library can also be used from C.
6. The 3D renderer sounds very nice, and I would certainly like to see it when you are ready to share it. Yes, it will be able to perform MUCH faster on the MEGA65 for various reasons. I would be very happy to help you optimise it for the MEGA65, so that we can show off what the MEGA65 is capable of in this area.
Keep up the good work!