If you are serving a crowd, you want a large roast.
A good joint of beef is great. And for a crowd, doing the ones with a bone still on it (a rib roast?) or a long sirloin roast, would be great for flavor.
A couple of legs or shoulders of lamb - good butchers will take out the bones and either stuff it for you or leave it open for you to stuff yourself.
A big pork joint could be nice too.
3 or 4 ducks, or a couple of geese, could be another way to look at it. Or 2 or 3 really good chickens (proper free range ones that have grown enough to have good flavor). If poultry is something you are interested in. (Or 12 little quails or partridges - 1 each!!)
The more important element is to think about how to make it a "special" meal - so does your joint need stuffing and what goes best in it (like maybe apricots in a lamb stuffing, or pieces of grated apple in a pork one), or a coating (mustard and seasonings on beef, garlic and rosemary on lamb, etc).
What sides will go well - some might be the same as you use for turkey, like roasted root veg with mandarin juice in the seasoning and roast potatoes. Whereas others may depend on the meat - cauliflower cheese goes really well with lamb but I don't tend to like it with pork nearly so much (or turkey for that matter!).
And your sauces. You probably want some regular gravy with the meat juices. But you may want something else - apple sauce with pork, a mustard sauce or béarnaise with beef, mint sauce with lamb etc.