A hybrid build would be extremely hard to make work. Its most likely possible, however:
1.) you still need to specialize in one or the other in terms of magic and strength levels which will throw of the balance of any hybrid build.
2.) By trying to use both strength and magic, you will lose power in both. For example, a player that specializes in magic will have a much easier time dealing decent damage by just using magic than a player with a hybrid build will. Same with someone specializing in strength. In order to use a hybrid build effectively; you’d need to have a magic that synergizes well with bleed effects (Not hard to do in all fairness) and you’d need to perfect the art of chaining together both magic and strength attacks while doing the things you normally need to do in a fight. This applies to both pvp and pve, but it will be harder to make a hybrid build work in pvp.
as for the spell thing, I doubt it. It was removed for a reason and I think that lost spells, lost magics, and primordial magics will replace this feature.