This merry band has a spell casting gap. (Although in 3.5 D&D at least, Rangers eventually get healing spells, and some offensive ones). The M-U in Goblin Hero was clearly not high enough level yet (and was lacking in personality), but eventually she will need to be replaced, if only to free up the Elf from providing artillery. Although I suppose they could always hire out independent contractors, as needed, for especially tough contracts.
In Pythonesque terms, a Tim would fill in the group's spell gap. While keeping to the Celtic theme, as a Druid. A Kobold Shaman would be funny and cute, keeping with the zoomorphic theme -- and add devastating ranged attacks -- but also typically Evil.
Yes, an MU character would balance the party nicely. Currently, the trio is not really optimized for beating up monsters and such. Aside from the elf, they're not even equipped properly for battle. This may change however... yes...
According to the earliest, Welsh-language sources, Merlin was actually a Bard, not a Druid. A Bard would keep to the Celtic theme, and, if geared toward offensive spell-casting, deal some real damage with Sound spells. But Sound spells don't get really lethal till at higher levels, in 3.5e, so they may be irrelevant, so far as this party is concerned. And I'm sure there are some non-Evil zoomorphic Shamans out there. But at least if you're going with a more anthropomorphic character, nothing beats an M-U for taking out those especially tough to get at critters/bosses.
The characters are not based on any particular individuals. I'm sure lots of things subconsciously inspired their creation. I'm not sure if there is such a thing as "pure imagination". Imagination is always contaminated with pesky life experiences. ;)
- Dennis, member of an autonomous collective