It is not mandatory to serve a starter with your Christmas dinner and if you do it's advisable to keep it small and not too filling as the main course is a big affair. I think the ultimate starter (and it's just my opinion) is a good smoked salmon, cracked black pepper, wedge of lemon and a couple of small triangles of buttered brown bread (real butter mind!). That's not always practical as some people are not so keen on fish but if you all are it's very easy, just remove the salmon from any plastic wrapping 1 hour before serving to eliminate the plastic taste. If you have a big party and a few people don't like fish then perhaps a soup prepared in advance and warmed through before serving.
Avoid any strong meaty dishes as this will be too similar to the turkey
Ultimately the decision on a starter is yours, consider the number of guests, the number of alternative dishes you need to prepare and space in the kitchen before going one way or the other. Also, consider the affect on appetites before the main course. It may be better to serve some canapes 1-2 hours before dinner just to push the appetites and keep people on the edge of anticipation!