It's a very interesting project but also very ambitious. Be aware that other similar attempts have failed previously (remember the first Mer?).

Also keep in mind that Debian is too big and not optimized for mobile use. Maemo has already done a lot of the initial work in that direction.

My suggestion for you, would be to start from an already working base, like CSSU Thumb (so you can be synced with fixes and features from CSSU) and newer toolchain, then go as far as you can without breaking the device's functionality.

There are several upgraded packages floating around the forum, some from Harmattan, some form Debian or upstream that could be easily added to that base. We have working (and better) replacements for common applications such as Worldclock, BusyBox Power, Hamster Filler, Open Media Player, improved Nokia Maps...

With not so much effort, and probably with more chances of receiving help from others, you could build a sort-of "Maemo Reloaded", keeping compatibility with most of (if not all) the already existing applications.

My proposal in such a project would be to keep as much compatibility as possible with existing applications, but without being as cautious as CSSU must be. Top goals would be to upgrade to upstream where possible and substitute closed source applications with OSS replacements where available.

You would need a new repository, in which applications could depend on these newer packages.

A second sub project could be the creation of a new fiasco image with saner partitions (i.e. ext4 and 5Gb for applications) and investigate the possibility of unoptification and decompression of root filesystem. But this can be made in parallel with the first approach.