TvP is in a funny state currently. It's not necessarily imbalanced, and if it is, most people will say it is against Protoss; but the match-up is still somewhat awkward to approach from a Terran's point of view. Since WoL, Protoss has developed a much stronger set of early all-ins, to the point that many Terrans are avoiding the classic one-barracks expand for difficulty defending it. A few still barracks expand, relying on excellent scouting and game sense to hold off the wide range of all-ins available to current Protoss. But most choose to either reaper expand, ensuring that whatever threats the Protoss might have in store can be scouted and held off with ease, or CC first into lots of barracks units, simply accepting the risk of an early all-in and moving on. Of course, there's always a few fringe builds, but those three openings describe the vast majority of TvPs (at least that I've seen).
Because virtually every single Terran build is still built around the classic WoL timing of hitting with a lot of barracks units, stim, and medivacs around ten minutes. It's somewhat reasonable to build everything around this timing; it was such a strong timing in WoL, and it's only gotten stronger from the improved HotS medivacs. Preventing the Protoss's third is the most important way to gain a midgame advantage right now, and speed medivacs allow the Terran to spread out the Protoss army so that three bases is too much space for a Protoss to defend. In other words, the ten-minute stim+medivac push is a good goal; but if getting to it is a little more difficult now, is there perhaps another midgame to aim for that is more easily defended in the early game?
Here is where the speculation begins. Marine+ghost is, theoretically, good against most of the early pressure options Protoss can throw out. The goal would be fast 2 bases with 3 barracks and a ghost academy. Add-ons would be either one TL/one reactor/one naked or one TL/two reactor, depending on precisely how the economy timings work out. Against a warpgate or warpgate/immortal all-in, ghosts can just EMP immortals, sentries, and stalkers to help with the hold. Against stargate, ghosts can EMP oracles (or theoretically, phoenixes and MS cores) and use their bonus damage against light to defend with ease. Probably the most difficult hold would be a blink all-in, but if scouted, the tech lab barracks could favor marauders instead of ghosts and, hopefully, defend without too much trouble. (I've never had a good idea of how to defend a blink all-in, but I generally have an easier time if I'm focusing on getting out units rather than teching to medivacs.)
But what's the new midgame goal, if not the ten minute medivac timing? Most Protoss tend to rush out some X factor to defend the ten minute medivac+stim push, most commonly colossus. And of course, marine+ghost struggles significantly against colossus armies, so there'll have to be a good alternative timing in order to push out in the midgame and take an advantage. Of course, it will be difficult to find out exactly how this plays out until the experimentation stage, but the most promising window seems like it would be to push before colossi can be out. That means pushing sooner, maybe at the 8 or 9 minute mark, and trying to get some damage done before the opponent's colossi come out.
-Take a fast natural expansion more quickly and safely than Terrans usually can
-Defend any incoming Protoss all-ins with greater ease than normal barracks openings generally are able
-Push out at 8 or 9 minutes with lots of marines and ghosts, hopefully EMPing the MS core before a nexus cannon can go down, and kill off probes in the Protoss natural. The marines might die, but try to keep the ghosts alive. Take a third base behind.
-For macro play, go ghosts, then double upgrades, then medivacs with the gas. For aggro play, go ghosts, then medivacs and perhaps a single engineering bay.
-Hopefully the fast third and early push can secure you an economic advantage. For aggressive play, follow up with ~8 barracks of MMM production, vikings as necessary. For macro play, follow up with 5 barracks of MMM production and a fourth CC.
-Ghosts might not be ready in time to defend some all-ins. It'd be nice to be able to gasless expand, then add on 2 more barracks, then get gas, then get add-ons for the rax, then get ghosts. But it might be necessary to delay barracks add-ons, or even to get gas before expanding, in order to get ghosts out in time. This could be a prohibitively high gas cost at such an early stage in the game.
-Certain gateway pressures might be able to contain this push at the Terran's natural, preventing a push out from the marines+ghosts and allowing the Protoss to take superior economy while medivacs aren't out. Medivacs give a MMM army the power to engage more than once, since stims can be healed again. Because of this, any MM army without medivacs is a one-hit wonder, so engaging the Protoss before reaching his base is going to be less than ideal.
-Blink all-ins will still be an issue. EMPing stalkers is great, but won't be the greatest help against a blink all-in, since they can just blink out and regain their shields again. Perhaps in experimentation it will turn out blink all-ins are surprisingly easy to hold, but I would expect this particular all-in to still cause significant issues.
All of these potential issues will be sorted out in the experimentation and refinement. It may be that there is no viable way to actualize this speculation into a real, workable build. But if it succeeded, it could radically change TvP. Currently Protoss depend on the fact that Terran almost never sends out attacks before something like ten minutes, so they use this time to expand, tech, and get out defenses just in time to defend the big push. Sometimes a Terran will toss out an anemic attacking force like, say, 6 marines and a widow mine early in the game, and the Protoss struggles to defend because it came before the Protoss planned to defend. If aggressive options exist at 8 or 9 minutes, Protoss must be ready to defend sooner, forcing them to tech less aggressively even if Terran isn't, in fact, doing the earlier ghost push. That means less stargate and DT play, slower weapons and armor upgrades, and faster colossi just in case the Terran is going to be aggressive sooner.
It'll be a couple weeks before finals finish and I get around to the experimentation stage. Perhaps by then the TvP metagame will have radically changed, and all this will be useless. But if you enjoyed this, and are curious how these ideas get fleshed out on the ladder, check out the blogs section in a couple weeks!