Before you give up entirely on your idea, ask the DM precisely *why* he will not allow your prestige class of choice. If he simply says "because I don't like it" or "because I said so", take a walk and don't look back. The issues with coming in as a new player on an old campaign is that you end up being the "odd man out" and are oftimes overlooked, regardless of what you play. However, if he gives you adequate reasonings and specific examples of what he doesn't like/want in his campaign, see if you can work together to come up with a concept that you enjoy and works for him.
That aside...
Yes, the party is "missing" a pure wizard, but if they haven't needed one by now, they probably won't. Here's my advice.
I would go with a Human Bard/Druid with maxed out ranks in Use Magic Device. You're pretty much the ultimate jack-of-all-trades this way:
double up on healing magic
double up on supporting magic
songs for support
shapechange when you need to kick butt/do fancy stuff
UMD for the arcane "blaster" items
druids have some good "blaster" magic besides
As for a "prestige class", don't bother. You're already going to be bogged down with lots of options and tactics without having to worry about when you cna use your uber-stuff from the prestige.
I don't know if this is the same, but isn't there a Shapeshifter prestige class in the Ranger/Barbarian/Druid splatbook that allows you to shapechange into any subtype of creature? Go for that one if he'll let you and you REALLY want a prestige class. *G*