Hi,
If i may, i would suggest splitting downloads to sections feature, like FDM has.
Its not usefull for free download slots as they usually support only one connection per IP, but premium users can use this feature to improve download speeds. Most of the servers do support sections anyways and download speeds could improve dramatically.
For example anyone who has fileserve.com account can experience some very low download speeds on separate connections lately. Sectioning using FDM improves download speed by 500% (really).
Of course, I'm aware it requires some huge changes in core and all the plugins, but it really is a nice feature to have :)