Your basic requests are all great. I currently use MC for video games, books, and comics.
1) Basic Cover Art Support
Though I have gotten used to the work around on my desktop, it is very annoying to do on a touchscreen tablet every time a file is added.
2) Sidecars
I would like them for marking books and comics as read.
Also when transferring files to another PC I wouldn't have to worry about naming conventions for Tag on Import.
3) The "Launch with External Program (default)"
This doesn't affect me personally because I'm not currently using a network, but there have been reports of the problem you noted.
I believe 1 and 2 would benefit users of SACD iso's too.
I would be extremely happy if just 1-3 were added to MC.
A couple notes about the advanced requests.
In-File Metadata Support
Calibre can scrape. It stores a opf file either inside or beside epubs(it's up to you).
Comic Rack can scrape. It stores metadata in "cbz" only by putting an xml file inside.
Is there currently any way to store scraped video game info outside of a database?
An issue is the number of file types users may have.
For comics:
cbz/zip/cbr/rar/cbt/tar/cb7/7z/pdf/djvu
For books:
AZW, AZW3, AZW4, CBZ, CBR, CBC, CHM, DJVU, DOCX, EPUB, FB2, HTML, HTMLZ, LIT, LRF, MOBI, ODT, PDF, PRC, PDB, PML, RB, RTF, SNB, TCR, TXT, TXTZ
For comics I only use cbz and for books I only use epub, but it's unlikely that's typical.