Suggested DG 2.5 row melodeon layout
WARNING: ONLY READ IF YOU'RE A BIT GEEKY ABOUT MELODEONS
There's lots of good music to be played on an ordinary 2 row 8 bass diatonic accordion, enough that you might never get bored of the possibilities available. However, for my purposes, which include playing in bands where I might not have any choice about the repertoire or key, it's important to be able to play tunes in a wide range of keys, often with accidentals, and to be able to play good left hand accompaniment for all of them. Some players go the way of having multiple instruments to play in different keys, but this would cause a few problems for me. A band may want to do a set with a funny key change in the middle (D to E, G to Bflat, etc.) and I might not have time to switch instrument. Some tunes have different parts in different keys and I need to be able to play those. Some tunes have accidentals that may not be workable using the chin end of a 2 row box. For these reasons, and others that are simply practical and logistical, I've always preffered having one box that will do everything I need it to.
This is the layout I've been using for the past 5 years or so. It's fully chromatic from low G (the lowest note in most folk tunes as it's the lowest note on a fiddle) up to the D two and a half octaves above (higher than most folk tunes will go). I find it works well for all the keys I ever have much reason to play in, which are the major keys of Bflat, F, C, G, D, A, E and minor keys of G, D, A, E, B. I know it can also do a decent job of Eflat and B major, and C and Fsharp minor, but it's rare that I ever need to play in these.