Sadly, I must admit, I haven't seen all of Canada yet. I plan to. Figuring out a Canadian itinerary can be daunting. There are so many options, and variables. It will not be cheap. For variables, time of year, would be an important one. I am going to say September is the best weather month of the year for all the provinces of Canada. In the East, the trees start to turn a kaleidoscope of colours, things are being harvested, and there is a possibility of an Indian summer in the West. Anytime in July or August is a good time to visit too. Expect lots of rain in the spring on either coast. Winters are milder in Vancouver.
Almost all Canadians live near the US border. 90 percent the stats say. This is also where the train goes. It takes about a week from east to west, and there's a whole lot of nothing for miles in some parts. To save time, I'd recommend flying some of the distances.
You will fall in love with the Maritime provinces and the people, best to fly directly there and rent a car, stay in B&B's and drive the coast to see Nova Scotia, PEI, New Brunswick, and if you have time Newfoundland. Then take the train to Montreal, stopping first in Quebec City. Spend a week enjoying the rich culture, sights and sounds. Toronto would be your next stop, train or fly there. Spend a few days.
Next is a huge decision, take the train across the country, or fly? Via Rail offers a $2000 for a sleeper berth for the 4 days. A flight with Air Canada will be no more than $500. With the train, you will see the vastness of our country, going through the provinces of Saskatchewan, Alberta, and Winnipeg, and most of Ontario. You can perhaps disembark in Banff and enjoy the beautiful Rockies for a few days. Lake Louise is a must. Radium Hot Springs.
Either way, If you end up in Vancouver, you will think you have died and gone to heaven. It is stunning and plenty of things to do and see, and there is also the option of a quick ferry ride to the provinces capital Victoria, which is also a treat. Tofino is a hidden gem. Whistler is a must. Go to the Okanagan for wine tastings and festivals. If you have a few days drive up the sunshine coast and come back down on the island. Lots of little places to explore.
People come to Canada to go north of Winnipeg to see the Polar Bears. People come in the winter to enjoy the Northern Lights or dog sleds, or ice hotels. All are extremely expensive because they are way up north but they are worthwhile ventures. I personally love the Yukon and find it fascinating to explore. You could also take a cruise ship from Vancouver and go up the coast of BC seeing interesting towns like Ketchikan and perhaps hop on the scenic train from Skagway to the Yukon.
I guess it all depends on what you enjoy doing. What do you want to do?