I feel like maybe going full warlock with a Gloom Pact Multiclass feat set might work best... though I feel Multiclassing IS underpowered considering you have to sacrifice so many feats and/or a paragon path.
Regardless of whether or not your builder lets you do it, that's not a legal option. You can't MC into a class that's a sub-class of your class. Multiclassing is not underpowered at all if you make good power selections - I can make Vampire or Binder into a build that many tables are going to have a lot of problems with for being overpowered and too good.
In general, hybrids are about 4 things:
Make sure you have 18 primary stats(you can break this rule, but it helps to know exactly why you're breaking it)
Fix AC. Make sure you have a role appropriate AC - 18 for Defenders, 16 for everyone else, +1 in many cases for 'tough', -1 in very limited cases. If it doesn't happen naturally, spend the hybrid talent feat to fix it. Unarmored Agility might be the answer, so might be wielding a +1 Aversion Staff in the off hand if Staff is an implement.
Know your role. Defender? Leader? Striker? Controller? Some weird mix?
Power selection. If you're good at picking powers for optimization, you'll do well here - in fact, it probably will more than make up for anything wrong in general with the build - a Sorcerer|Wizard might be a very odd build, but Flame Spiral+Prismatic Burst+Wizard Dailies+Demonskin Adept ought to compensate for a lot. If you're bad at it, it isn't a good choice.
Poachable Powers