I don't care about Vancian particularly (though I love it in my D&D and Pathfinder, and sitting there staring at the spell selection screen can be oddly enchanting), but I really want Obsidian to come up with something other than mana pool or (*shudders*) powers with cooldowns.
For me, the biggest drawback of Vancian system is that low-level spellcaster options are extremely limiting. Pathfinder tried to address this with at-will cantrips and school/domain-specific abilities, and it worked to a certain extent. It's not that all classes should have roughly equal possibilities at every level, anyway (I'm looking at you, 4E). Of course, Obsidian is not making a D&D/Pathfinder game, and neither I want them to make one. But this spirit and general approach to magic is where my heart lies.
If anything, I want complex system that provides both strategical and tactical depth, allows for specialization and spell variety beyond DPS/crowd control/buffing, and has the potential for a combinatorial explosion of choices. And above all, magic should feel magic, not a game mechanic device. It should be reasonably balanced and convenient, but not built for balance and ease of use.
I am somewhat opposed to the systems that gradually reduce magic efficiency while spellcasting resource (mana, stamina, whatever you call it) dwindles. This seems like an invitation for potion chugging (or equivalent) and if it's not an option, your imminent failure just becomes slower, more painful and boring.
Use of long and complex rituals (time resource) for potentially miraculous effects is an interesting idea to explore, but I wonder how it will work out in RTwP.