I was planning to post all this over in the feasibility forum, but you asked.. lol
I guess I can copy and paste all of this over there after this... 
It's going to be a user programmable MIDI controller/effects loop switcher.. There will four switches for switching loops in and out, a switch for choosing amps, and a switch for chain mute.. There will another section with a select button and switches for program up and down.. There will be an LCD readout with buttons for editing/programming, and a rotary encoder..
The unit will have three modes;
'Manual Mode' (the 4 switches control the loops and a red LED lights above the switch to indicate a loop taken out of the chain, the up/down switches move through programs up and down one at a time, the select switch makes the MIDI controller apply the selected program),
'Program Mode' (the unit allows manual input of which patches are in a 'scene,' which scenes and order are in a 'song,' and which songs are in a 'set'.. then you select which loops you want in the scene manually, select which program changes, channels, etc are in the scene, and then you can save the whole shebang), and;
'Run Mode' (the unit then uses the 4 switches that are used for inserting removing a loop from the effects chain, for selecting one of the eight scenes.. there will be a blue and a green LED below each of the the switches to indicate which of the 8 scenes is chosen, ie 1/5, 2/6, 3/7, 4/8, and any loops that are switched out of the chain by the program scene will have it's red LED above the switch lit)...
There will be an RGB LED above the 'select' switch to indicate which Mode the unit is running in, sometimes it will flash to ask if you want to select or to save the scene (depending on mode)..
It's be nice to be able to include a tuner powered by the Arduino, and the ability to save/load scenes, songs, and sets to a flash drive.. I want the user to be able to enter names for the scenes, songs, and sets... I also want to integrate a buffer into the guitar chain...
That's basically what I want to do.. You can see that I'm gonna need a lot of I/O, and as much memory as possible.. I don't know if even the MEGA 2560 can handle all that, and if not, I might need to have it talk to another one..
I know this is going to be a long project, and I will learn a ton doing it.. In the beginning the programming will be pretty easy, but once I want to add the UI and do all the parts with scenes, songs, and sets, I'm thinking it will be a painful learning curve.. But that's okay... Plus, I'm sure there are others that have attempted/accomplished similar pedal board controllers so I can draw from what they did too..
I also bought some LCC110 ICs to use as relays, should I decide to do that.. I was already planning on using three of them to do a delay mute of the whole chain (a few milliseconds) whenever a loop is switched or a guitar amp is chosen, using them as the relays for both the chain mute and the guitar amp relays..
Sounds like I should plan on using them for all switching.. I also have relays.. one is a 4 relay board populated with DPDT relays, LEDs, and some resistors, transistors, etc.. Said it was for Arduino when I bought it.. Pretty sure it supplies power for the relay transformer separately (hence the terminal for 5v)
I bought a 2 relay version too, to use for the guitar amp and mute switches (but I had already decided to use the LCC110s for that so that I can do the delay thing, to avoid pops and clicks when switching loops)
This is the 4 relay board I bought
This is the 2 relay version I bought
I also got these Hex inverters to use for the MIDI outputs
These are the faceplates for the loop switching section, and the MIDI section (there is room for the LCD and programming switches to the left of the up/down/select switches on the MIDI section..
