Quite frankly, our power armored friends don't need much. Their troops are durable, and their (good) toys are incredibly cost effective. However, there are definitely areas where they'd appreciate some help, and the most obvious is with their troops. Power armor is expensive and scouts really don't save enough points for those stats that they lose. Not only that, but bikes are even more expensive still. So, for those facing LoW and/or intending to run bikes, finding an out to cheaper troops is definitely something to look for.
From there, it's important to consider where and how our army will be playing. Space Marines are an aggressive army that typically needs to sit mid field to maximize its firepower. This means that if we choose to consult another codex, it should be for either more aggressive, assault based options, or longer ranged fire support options. Assault forces need to be as fast or faster than your marines, who themselves have no excuse for not being able to declare turn two assaults if that's the kind of game you'd like to play. The additional fire support is more of a tricky proposition, given the dazzling array of potent weaponry available to us. That said, we are lacking some degree of potency in low AP blast weapons. Plasma cannon devastators and vindicators exist (as well as the chapter master), but those are about the extent of our options in that area. That said, as the game has moved to include more monstrous creatures, as well as hordes which AP 2 is wasted on (cover save notwithstanding), these options may be less crucial in your area.
This touches on something else to consider. The Space Marine codex appears to have a large vulnerability to Chaos Daemons, especially if you aim to utilize grav weapons in an attempt to hate on other codices big nasties (bugs, knights, tides). Certainly, bolters hose through daemonic troops just about as well as anything, and that's absolutely an out you can play to (mutually assured destruction for the draw!), but assuming that you'd rather not have to fight hard for such a draw, volume of mid to high strength fire to cope with those pesky daemon monsters will absolutely need to factor into our decision.
So, with the full compliment of the Imperial forces behind us, as well as access to the Tau, and the Eldar, to whom should we turn?
Past that, Guard and Tau are probably the best contenders for allies. Guard offer you suitable henchmen replacements, should you have elected not to include Coteaz, and Tau offer you Fire Warriors, as well as their impressive array of suits (yes, including the suit commander) which serve as replacements or supplements to your Sternguard or Command Squad. If you're playing mechanized marines, I would personally advocate Guard. Leman Russ and/or Chimera hulls work very well in concert with Predators and Rhinos. But for biking or drop pod marines, Tau offer a low-armor gun line to minimize outlets for enemy anti-tank fire. Do note that if you did happen to bring any of your Inquisitorial models within 6" of the Tau, it's my understanding/assumption that one or both of those armies would suffer from the "One Eye Open" rule. That in mind, if you utilize henchmen, consider allying in Farsight Enclave Tau, as their suit troops are mobile enough and offer fire at a range which allows them to keep their distance from any Inquisitorial forces.
What of the elves then? Eldar simply aren't cheap enough to displace the other options unless you brought Coteaz and henchmen and even then, the few toys you could opt to bring aren't that special. Wraithguard or Wraithblade troops are even more expensive than your Marines, jetbikes don't offer enough for their small points savings any more than scout bikes do, and bringing wave serpents ensures your rhinos or podded marines bear the brunt of the enemy fire, which is not what we want. The same can certainly be said of the Wraithknight as well.
As for the prospect of consulting other marine armies, I would advocate against such, forbidding a compelling need to bring a rune priest to hose enemy psykers. Other marine codices simply don't offer anything that we can't do on our own. Unless you're talking Grey Knights that is. If you don't need Coteaz to cast as a Brother, then he can certainly bring some Knights (including a dreadknight or troop terminators), to provide you with anti-daemon measures, as well as warp quake. They don't necessarily fix our points problems without henchman use though, so that's worth considering.
Once again, the Space Marine codex is extremely good about being self contained, which is a huge boon. You don't need to fork over the cash to buy another book unless you really want to, and there's no compelling need to start an allied detachment to build it into another force. At the same time, that same flexibility means that for those metas which need just a little something extra, or if you'd like to build another army, you can play with the new models until they're supported well enough to stand on their own.
Be sure to join me next week as I walk through some basic list paradigms, personality quiz style!