Of the potential opponents Luffy may face, so far, we have Smoker, Aokiji and Lucchi. These are opponents where we know (or at least have a partial idea of) their abilities. We've also seen so far in the manga/anime, Blackbeard, and the other Marine admirals, but we have no idea what their abilities may be, so it'd be pointless to discuss ways Luffy may defeat them. So for those 3 previously mentioned, where their powers have already been revealed, how will Luffy defeat them? What for example might Luffy use to counter, Aokiji's ice, or Smoker's smoke, etc…
A possible counter to Aokiji's ice abilities, might be salt (an ingredient many of you may have used before to melt ice during the winter). Luffy might use salt/sea water to defeant Aokiji, but that would, IMO, end up being too gimmicky and lame, since Luffy already used water (albeit this time it would be salt water) to defeat Crocodile. Another (obvious) counter to Aokiji's ice ability, of course, would be fire. So how might Luffy utilize fire to defeat Aokiji? One cool possilibility (cool at least IMO), would be for Luffy to create a punch so fast, that it ends burning up the air around his fist, like how an meteroid burns up once it enters the Earth's atmosphere. It might be possible for Luffy to stretch his arms far enough (perhaps even into space!) to give his punch enough velocity for a burning fist. And since fire is an obvous counter to ice, a flaming fist attack would have the added benefit of suppressing Aokiji's ice abilities so that he doesn't crumble into ice pieces and reassemble.
As for Smoker, the only counter I can think of for smoke is air. A way for Luffy to utilize air to defeat smoker, may rely on the same principle I discussed above for defeating Aokiji - and that is a really fast punch. However, this time, that punch would be used to massively compress air onto Smoker's body, causing an enormously damaging impact, without Luffy's fists actually touching Smoker's body. Also, the punch can't be as fast as the Gomu Gomu flaming fist attack discussed above, since that would end up burning the air around Luffy's fist rather than compressing it. So it has to be fast enough for a compressed air attack, but not so fast that the air starts to burn.
As for methods that may be used to defeat Lucchi, this has already been discussed in another thread, so I'll not touch the subject here (plus I'm feeling a bit lazy and want to save myself some time typing :D). But, please feel free to discuss possible ways to defeat Lucchi in this thread anyway :D.
If anyone else comes up with other possible counters/methods for success for the above opponents mentioned (or other possible opponents not mentioned), please discuss them here. I'd very much like to read your ideas.