As a twenty something person surrounded by similarly aged people in a culture that is rampant with marriages I have seen my fair share of engagement photos. These are my rules of engagement photos followed by examples from commercial photography which is allowed to be avant garde bc no one's getting married.
-No touching of stomachs, mostly the lad touching the lass', we needn't be distracted by whether or not we are being invited to a shotgun wedding.
-Both bride and groom need to be fully visible. The biggest violation of this law is 'candid' whispering shots where the groom is being oh so secretive and is completely obscured by the bride.
-No looking pissy. You're getting married, you're in love, even if it's super hipster or trendy to look super pissed at everything the announcement of your intent to marry should not cause your loved ones to walk on eggshells around you.
-No piggy back rides. Groom carrying bride, bride carrying groom (original), this is just dumb and awkard.
-As few props as possible including signs, umbrellas, balloons, flowers etc.
-The photo is not about the ring. Showcasing the engagement ring as the focal point of your photo is tacky and unnecessary. He bought her with a diamond, whoopty doo. If your guests want to see the ring they need to buy you a present and come to the reception.
-Coordinate outfits but don't be matchy matchy. Just look good people.
-Be careful about on the ground/lap poses. These just get dicey when there are too many knees, groins and elbows in a shot.
-Pick a relatively neutral location. Pretty meadow, great. Waterfall, perfect. Abandoned junkyard with a tire fire? Distracting.
and lastly
-ABSOLUTELY NO KISSING. seriously, I mean it.
The morale of the story is that an engagement photos should...
a) show how attractive you and your fiance are (meaning fully visible shots of both w/ normal faces)
b) show that you are in love
c) look great on someone's fridge and not attract negative attention
All in all an engagement photo should look like a Brooks Brother's ad. If you choose to do any of the above shots for a personal collection that's fine, but an engagement photo serves a very specific purpose and therefore should be as simple and straightforward as possible.