We have very few players to play with nad need alot of things covered, so I'll tell you what we do. We pick a class. We take all their class features and at least one at will from this class. All other power choices come from about any class as long as it makes since for your character (themes, background, etc.) If you play a fighter and your mother was a wizard. She always wanted you to be a wizard, but you only ever managed to learn one thing (now chose an at will or encounter from Wizard). Unfortunately, your fighter isn't very Intellegent and it kind of shows when you try to hit with the power, right? That's just the price you pay for powering up outside your class, you may not be as good at the extra things you want to do.
Multiclass feats are still available, but they really only give you a class feature and new trained skill.
Weaknesses from ability scores can usually be overcome with feats. Took wizard, want fighter skill, not strong, take Melee Training. Took cleric, want warlock spell, not very durable, take multiclass to gain implement to help with power or damage feat, like Astral Fire, that is appropriate for the spell.
It's not perfect but it works for us