Some ideas regarding spell casting:
*A spellbook to keep track of everything you collected so far would be awesome. I found myself dragging a log of scrolls around, since I couldn't remember everything, but also didn't want to get a spoiler from the internet.
*I liked the idea of having to collect runes in RTC. Another idea would be an option to play with randomized spells. Sure there's trial and error to find out all spells again - but people who'd do that simply wouldn't use this option anyways.
*Concerning buffs, some spells like rage or speed would be far more interesting (and less of a hassle) if they were cast on the group instead in bottles. This would also allow to have the caster level a direct influence on the spell effect/duration, enabling a whole new class build (or at least an interesting tradeoff for mages): a supporter. A buff bar would help with that of course (have a look at any MMO for inspirations, I found the buff list Dungeons&Dragons Online usefull and intuitive), otherwise the portraits would be full of icons.
About combat:
*I liked the enemies who did surprising things: charge, turn around and swing at the same time, walk sidewards and maybe more. I hope you have some more interesting ideas for that, that makes fights really interesting (and also a lot harder) compared to the good old circle dance. For example a stomping monster with a "melee all" attack or monsters pushing you around (there was this nice fight in DM 2...).
*To make combat more versatile, I hope you can include some active defense like blocking/parrying or debuffing. That was in DM but somehow never worked. In LoG it's sometimes very hard to avoid damage at all and I found few difference between armor and evasion fighters. Surely my defenseless ranged rogue perished even quicker at the front, but for my first row melees the main defense concept was drinking lots of potion.