What size are you looking at?
Staying the same or going larger?
There are 1000's of alloys available (if you want to look at alloys sites).
I guess so long as you keep within 5 x 110 pcd; et range of 33-39 and 7.5 - 10J (depending on how big you're going); centre bore is ?60.1 - the world is pretty much your lobster.
Don't be tempted by smooth talking alloy wheel outlets that want to flog you a wheel with 5 x 108 or 5 x 112 pcd and the 'yeah, mate, wobble bolts'll sort that out'. Not safe (IMO) and not a good idea.
If your Elite hasn't been lowered I'd say you could go up to 19" quite comfortably. I personally think 19" look awesome on an Omega. My estate was lowered and I then went to 19"s. I was delighted with the look of the car, changed it completely (as far as I was concerned). However, with so much wheel and not as much rubber you feel every damn thing on the road (including crisp packets and sweetie wrappers). Handles beautifully but the comfort of driving on rocks. Elite suspension out the factory is quite comfortable I understand................
I'm back on 17"s.
What do you want to do - change the look? Fit wheels from a car you've seen and like the look of? Deep dish staggered fitment? Go bigger? Go smaller?
Good luck whatever you decide!

Matthew