Here's a really interesting question.

What happens if either or both parties end up having brokered conventions?

The thought gets kicked around a lot, but it has the real potential to happen this cycle.

On the (R) side, there are a ton of candidates still in the race. It isn't at all difficult to see a scenario where none of them secure enough delegates to win outright. Further, not all the states are winner-take-all and pledged delegates aren't really bound to a candidate once the convention starts(but they virtually never go rogue).

On the (D) side, it gets even more interesting. Not only are their delegates not bound to vote for a candidate, their party rules don't even say that the party has to assign delegates based on the results of the primaries at all. If Sanders wins enough delegates to in theory get the nomination but the party doesn't want him for whatever reason, they're free to give it to HRC.

How much fun would it be to see a general election consisting of Clinton (D), Cruz (R), Trump (I), and Sanders (I) on the ballot?

That would probably see the whole thing headed to Congress - where the House selects the President and the Senate selects the Vice President and they need not have been partners on the ballot or even members of the same party.