1. Two guys liking the same girl. But these guys should be intimate - no homo (best buddy, childhood friends, oath/sworn brothers, etc). And MC is one of those two guys, or the heroine is not the main protagonist.
2. Variation of #1: MC's crush is liking his buddy. And yes, not another way around.
3. Hetero friendship which turned into love. Again, I know this concept been overused, but I've barely seen those in which the hero and heroine are/already in the comfortable "friendship" zone and one of them (preferably the male) should make a call whether they'll go further or just stay in their comfort-zone. Few examples are: Amagami Kaoru arc, when both Kaoru and Yuuichi are already close and comfortable with each other and they're wondering if it was necessary to change their relationship, even after the "official dating" thing they're still wondering if the change was actually worth anything since their dynamics are still pretty much the same. Another example is Koitabi True Tours Nanto 2nd arc told about two childhood friends.
The bottom line is MC (Male) should make a call whether he would choose his friendship or his love. And yes, limited to hetero-only and male MC only. Sorry if it sounds sexist. I never meant that. Its only because I've seen lots of female on female friendship (And/or shojo in general). And no gender bent! Gender bender only allowed if the one who got bent (mostly protag) didn't get any identity crisis as opposed to Boku Girl or Idol Pretender. I only remember a manga (possibly a oneshot) which did it right (I mean, like what I've requested) when the boy got cursed to become a girl and he almost got NTR'd. And his crush is his childhood friend (I forgot the name, sorry for long exposition).
So, any idea?
Lame idea... 
