Ok, so not at my home computer so I can't tell how many rifts I've done. Probably around 50-100. While I'm not quite burned out yet on them, I do feel they need a bit of sprucing up in future content patches. I could definitely go for some really wacky gameplay in rifts since Orek kind of touts them as being in a different dimension.
Some ideas I was thinking about (this is all random):
Challenges -
Starting with a conduit pylon buff and trying to trigger the rift boss to spawn. If you can't do this in time then this will result in a FAIL state (as in, you can't cause the rift boss to spawn) doesn't close the rift for you, you can still kill the mobs if you want and you still get the XP/gold rewards from orek.
2nd rift boss - Maybe a small chance to spawn a 2nd Rift guardian while fighting the first? It could spawn randomly at any time as soon as you engage the rift boss.
A rift where if you stop you start taking periodic damage, so you try to be on the move as much as you can.
Soul Swap - Starting as a treasure goblin. You get 150% movement and immunity to all CC except walls and slow. Each time you get hit you lose some loot. If you can make it to your goblin portal then you get to keep everything in his sack. This should be highly rewarding. When you make it to your portal the treasure goblin drops his sack and you get transported back to your body where you can then finish the rift as normal. So you only get access to the loot in the sack, anything that dropped before hand gets magically eaten
Or your character can spawn 3 times the size they normally are and can smash through walls (like those demon tremor things in act 3) and you're immune to waller, vortex and slows and each hit has a super high chance to perform a knock back.
You spawn half as small and can pass through enemies and have 90% dodge with a buff to IAS. Damaged reduced slightly.
The rift level is entirely underwater and results in odd ball physics effects (totally aesthetic rift).
You spawn as a completely different character class.
Your follower is replaced with a duplicate of yourself, except you can't change your class skills.
I think the rifts that have challenges attached to them should have some sort of "handicap" magic find buff to alleviate any frustrations players might have. Any other ideas? (also not quite sure how some of these would play out in groups, some just might have to be disabled)