There are different roast dinner with different accompaniments.
All roasts need a roast meat or veggie equivalent. Most would be very disappointed with roast potatoes and gravy is almost always served unless it's a small meal and no-one likes it.
Beef - yorkshires and horseradish sauce
CHicken - stuffing
TUrkey - stuffing, pigs in blankets and cranberry sauce
Lamb- mint sauce
pork - cracklings and apple sauce
Lots of people love yorkshires or one of the sauces and have it with all roasts, but I don't think you could be surprised if you didn't get yorkshires with chicken or meats other than beef.
I'd say you'd want 2 veg but am flexible on what they are. More choice of veg always good - can be roasted veg, carrots, peas, broccoli,cheesy leeks, cauliflower cheese, green beans, sweet potato mash,msweets mash.....whatever you like really.
Fine to have mash or boiled potatoes as extras. Most visitors would be disappointed if they were instead of roast potatoes though.
And for me, the meat is my least favourite part. Would never say 'no' to pigs in blankets though!