I think that would make an excellent mode. Druids would definitely get a much needed buff to their spells.

A few of the "big mods" add some of them, but unfortunately, you can't add the spells without adding the content.

Druids already get extremely powerful spells in BG2, a spell which grants immunity to the next up to 10 physical attacks, a spell which turns enemies into helpless blind victims even if they make their save (if not they die instantly), mass resurrection, ...

Adding the "water elemental" shapeshift would be better because shapeshiftinmg is something only druids can do.

Hmm there is a spell pack mod which adds new spiffy spells to the game, might be useful to you. Find the link on this web page (http://www.sorcerers.net/Games/BG2/index_mods_hosted_items_spells.php) :