Back to the alternative pairings we go, then!
Considering the amount of 'tension' between basically all the characters of Gundam Wing, I think just about any alt. pairing could be made plausible. Having this huge fanbase *cougharmycough* of smut and sap craving fangirls and fanboys, complete with their own hierarchy and propaganda machine *coughRight,Wicked?cough* helps too.
Many ladies here mentioned that they have no problem with pairing Relena with any of the guys, but want Heero to stay exclusively dedicated to Relena. Of course; we're the feminine party, we identify with Relena. We don't want 'our' guy to run off with someone else. This limits our fun when reading alt. pairings, though, because Any Male x Relena is only a fraction of the Any Male x Any Female pairings possible in Gundam Wing.
I don't really have that 'problem'. I think of it this way: it's not
Heero I pair up with another girl, it's
Hilde/Catherine/Middie/Dorothy/Lady Une/Noin/Sylvia/Sally/Meiran I pair up with Heero.

See the difference? Though I (by far) identify with Relena the best of all the GW women, this enables me to get a full perspective of Gundam Wing pairings. Full appreciation of all the characters, and their romantic possibilities. No limit to the madness, whahaha! *cough*
My personal favorites - as of yet - are:
Relena x Duo. I see Relena as apt to supress, or forget about, her fun loving, spontaneous side, and Duo would really help keep that part of her alive, while she would provide stability and unconditional love when he needs it.
Heero x Catherine. While I'm not sure about long term commitment between the two, I really think there could be some form of attraction between them. Cathy is so full of life, so free, she'd be like a butterfly to him, and the little boy in him would be lured out of the Perfect Soldier shell to chase it. (I?m writing a fic at this very moment with his theme, please don?t steal it! Not until I?ve posted it, anyway. ;-p )
Trowa x Lady Une Because they cater to some... slight fetishes/kinks of mine:
1) The uniforms;
2) The hair;
3) The presence of guns (...don?t ask)
4) They look good together.
Treize x Noin would make for great angsty lemons. Because Zechs can be an inconsiderate bastard that should be chained to Noin?s bedpost and whipped into submission.
Quatre x Noin. My new favorite.

Quatre?s crush on the older woman, oh yeah... I so have to try that one out!
I never read GW Chara x OC, because I simply can?t relate the product of another fanfiction author?s imagination with the GW canon (despite the fact that I'm co-writing a story that has just about every single GW character paired up with an OC by one of the five authors... heh). Looking for the few masterpieces among all the junk is too much trouble. Crossover pairings are cool, though. They can give a whole new dimension to the characters, that you?d never be able to achieve with the other GW characters.
As for threesomes... no prob whatsoever.

I don?t mind smut, as long as it?s well written. There?s one thing I?m curious about, though; I can?t decide whether I like the thought of Male x Female x Male better, or Female x Male x Female. What you think about this, everyone?