Any plans for a lightweight client? The idea looks interesting but the requirement that users must download the Bitcoin blockchain might turn off some people from getting involved. Is this a limitation inherited by all BTC sidechain coins?
Yes, there are plans for a lightweight client, and plans for making it possible to point Credits to the Bitcoin working directory, so that it can read the required data from the already installed Bitcoin client, if one exists.
Unless the required information about the Bitcoin blockchain can be acquired from elsewhere, for example an online resource, then yes the Bitcoin blockchain must be downloaded to run a sidechain.
There are ways to shrink the storage requirement or completely get rid of the Bitcoin blockchain download. These approaches all involve trade-offs, for example disabling the claiming part in the client.
There are however plans on getting rid of the Bitcoin blockchain with a specialised client. Unfortunately no time plan can be given yet.