I am always up for new customisation options, bu on hybrid classes in particular... It could work, but Dragon Age is a party-based game that uses distinct roles and so on, so a straight-up hybrid wouldn't really work. In Mass Effect the other squadmates are far more "complimentary" to the main skills of Shepard, at least compared to DA, and even then in Mass Effect 2 the whole "hybrid" thing is basically dropped. E.g. the Infiltrator is less Combat/Tech and more "sniper", and the Vanguard has a very different playstyle to the Adept or the Soldier.
A stealth mage, for instance, does sound awesome. Dare I say, incredibly so! The key question is, however, what's it actually going to bring to the table? It can't just be a rogue with a magic flavour ("I'm not stealthing, I'm turning invisible!"), but it also can't be a mage with a couple of rogue abilities tossed in or a rogue with a few fireball spells.
The other alternative is to just drop classes altogether and give you a wide bunch of abilities to choose from yourself, an interesting solution but one that's probably not going to be used any time soon. I don't actually have anything against doing this, but we can only dream.