I play the Ruy Lopez
Transpositions in 1.e4 e5

I'm an e5 player, and I find there to be very little transposition compared to other openings. I postulate that it's because both players are being assertive, and their respective centers are under immediate attack -- there's not the time to screw around with positional subtleties as you can in queen-pawn or flank openings.
In those little variants in the Max Lange/Scotch-type games, if White starts getting too cute, Black can usually equalize by playing Nxe4 followed by d5 (often forking an Ne4 & Bc4).
Rarely, you might run across a Vienna player who transposes to/from some King's Gambit variations.
One could conceivably transpose from some Ponziani variations to Danish/Scotch Gambit, but Black would have to be playing pretty passively for that. Again, rare to find a player playing that opening anyway.
That's about all I can think of. Against e5, most people play 2.Nf3 with 3.Bc4 or 3.Bb5; occasional 3.d4 or 3.Nc3; occasional King's Gambit. It's pretty straightforward chess.
After about twenty years of playing chess, I'm thinking about starting to play 1.e4 e5 as Black. Not everybody starts with that move :-)
The Ruy Lopez is well known, and I know some things I want to play against it. But, of course, people will be trying all sorts of other lines.
What concerns me is move order tricks. It can be hard to keep track of all the possible ways that certain variations come up! One moment I've decided I don't have to learn some variation because my reportoire avoids it, then it turns out it can be reached depending on my choice of defence to 2.Bc4...
The Max Lange is a good example:
1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.d4 exd4 5.0-0 Bc5
1.e4 e5 2.Nf3 Nc6 3.d4 exd4 4.Bc4 Bc5 5.0-0 Nf6
1.e4 e5 2.Nf3 Nc6 3.d4 exd4 4.Bc4 Nf6 5.0-0 Bc5
1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.0-0 Nf6 5.d4 exd4
What I'm looking for is a large list of examples of this sort of thing after 1.e4 e5. Who can give some more? :-)