As I am currently also working on a C64 project, I keep having an eye open for new expansions to my C64.
Very recently I got aware of a new cartridge for the C64 named "BackBit".
The homepage of the expansions is here. And of course they have a shop. They do also have a forum, of course.
Below please find an overview that I grabbed from ebay, actually. Seems pretty advanced.
FREE UPGRADES FOR LIFE!
Combines the best features of the 1541, SD2IEC, EasyFlash, Pi1541 and Ultimate-II+. See chart in images for comparison. New features being added all the time through firmware upgrades.
Compatible with unprotected D64 disk images, and most game conversions designed to be compatible with IDE64 (when loaded as D81). Built-in instant loading so a fast loader is not required.
BackBit C64 bare cartridge
USB Cable (for firmware upgrades)
This cartridge features:
Instant loading from D64, D71, D81, CRT, PRG, P00, T64 and BBT files
Disk access supported by KERNAL emulation
(NEW!) CRT <= 128KB fully supported
CRT > 128KB supported by emulation
Preliminary EasyFlash CRT support
Up to 4GB of data per program via BBT file format
Unlimited firmware upgradeability via USB (cable included)
“Rip & Burn” disks on your 1541
Replaceable Real-Time-Clock battery
Fully powered by your C64
Works on C128 in C64 mode
Proven compatibility with most C64 revisions