One huge weakness with Rand Paul that Obama has also is that he is a senator and has no executive experience. This can lead to cronyism, terrible decisionmaking, and incompetence. Andrew Jackson would be another example (save that one exception we admire). This gives me a little discomfort and would make Gary Johnson a stronger candidate. I am still annoyed at him for not running for Senator, but if he ran as a Republican*, I could support him.
Other options may include Scott Walker and Paul LePage. The former has more establishment support and moderacy, while the latter is a bit more radical and polarizing. Both are in crucial and tense reelection campaigns that soon go to the polls. Reelection would strengthen their candidacies greatly. Walker already won a recall election, so a proper reelection would make him very formidable. LePage faces a three way race, which dampens his ability to claim a big victory. In the end, libertarian governors are not easy to come by and even those two are not good on key issues.
A goal for libertarians should be to work on governors races in key states: New Hampshire, Maine, Minnesota, Iowa, Wyoming, Montana, Washington, Colorado, Alaska, North Dakota, Vermont. Work on campaigns, get the right candidates in the primaries and win. Unfortunately, almost all of these are small states or marginal states, but thats not insurmountable. With a good crop of libertarian governors, the White House is not much further off.
Since it is a bit of a hopeless venture, it may be better to find a moderate but small government person (Walker/LePage) and then give them a good congress. They will support libertarian bills handed to them, but they would also support general establishment conservatism. Elect a sympathizer to the White House and libertarians to Congress.
*Third parties dont have a hope in hell for the presidency at this time, so supporting such a campaign is idiotic, with the rare exception of a Ross Perot type, ie a billionaire or a cult leader (eg Ralph Nader & Ron Paul).