Mount

Came up with an idea for mounts in magic. Ride [cost] means "[Cost]: Unattach all creatures from ~this~ attach target non-mount creature you contorl to ~this~. Play this ability only when you could play a sorcery." and "~this~ gets +X/+Y where X is the power and Y is the thoughness of creatures attached to ~this~" Additionally, you hafta change the rules so that creatures attached to permenants don't unnatach, but instead creatures attached to things can't attack or block.

Riding Bear 1G
Creature - Bear Mount
Ride 1G
2/2

Back

Well my computer expolded, but now I'm back, writing out my magic thoughts till I leave. With Lorwyn they're introducing a mechanic that is as close to substitution as they've done. Whta's substitution you ask, well let me tell you. Back when Urza's saga came out I came up with a mechanic called subsitution. This is the original card I came up with:
Pack of Goblins 2RR
Creature - Goblin
Substitute, 1R - A wall you control
3/4
The idea was that the wall that you thought was a buch of wood was actually a pack of goblins. I immediately realized it was incredibly versatile. It had several "handles" you could tweak to get new permutations not to mention Subsituting opponents cards and "Building" legends. The people I told about it all liked it, and it even went into the last HGML set made before it dissolved.
I sat and watched magic sets come close to replicating it, from gating to morph to flip card, and now champion. While it is the closest to my original idea I'm hoping that it goes the way of offering with a small showing and a luckluster response. Mainly because it's not as good as substitution.