Many specialist collectors reach the end of their specialties fairly quickly. Two constraints cause this. First, most specialties, such as US stamps, have at most a few thousand varieties that are needed to complete the task. Most of these are quite inexpensive and easily acquired. Then, the few stamps that are left can be very pricey, and so most collectors often find themselves soon at the point where there is little that they either need or that they can afford for their collections. When this happens, these collectors often look for another specialty. But if they choose another country to collect, they soon find themselves in the same bind they were in with their original specialty—the new collection is soon nearly complete and the stamps that they need still beyond their budget.
A fascinating solution to this philatelic conundrum was produced by the Scott publishing company in 1940. 1940 was the centennial of the first postage stamp, and Scott issued a hardbound, brown stamp album that contained spaces for every stamp in the world issued up to that time. The album was well produced and was extremely popular in its day—so much so that even 70 years later the album can still be found in like new quality with only a few stamps mounted in it, usually for less than $200. The album gives you the opportunity to collect the world to 1940 and has spaces for every country complete as listed by Scott. This means spaces for tens of thousands of stamps. Some of these stamps are rarities, but the vast majority are quite inexpensive, much less than a dollar each. The challenge then for the collector becomes finding these stamps. Many specialist collectors that I know still have their fine and expensive specialized collection of Stellaland but begin to get most of their philatelic satisfaction from their Scott worldwide album.
A few cautions are in order. First, Scott has produced many editions of worldwide International albums; most aren't very good. The one that you want is the hardbound, brown 1840-1940 album, about five inches thick when it has no stamps in it. Accept no substitutes (you may find that finding a nice copy of this album is harder than finding most of the stamps to go in it). Second, because the album is printed on the front and back of each page, the album begins to bow as the thickness of the added stamps forces the center of each page outwards away from the binding. Many collectors like this “acquired look,” but it does make the album a bit difficult to store as it gets thicker. This is hardly a problem in a collection of thousands spread out over many albums but can be a serious issue in a collection of tens of thousands in one volume. Consider only collecting used stamps. Mint stamps are much thicker because of thickness of the gum is usually equal to the paper itself.  Also consider buying some packages of high quality hinges and not using mounts for your stamps in this collection. Mounts quickly create additional thickness issues, and, because the high quality paper that Scott printed these albums on is very thin and porous, mounts are impossible to remove for remounting without tearing the album page.
These concerns sound daunting but are really minor compared to the pleasure of making a collection that is easy to get to (just pull down one volume from the shelf), offers all the pleasures of our hobby (one week you are collecting the stamps of Canada and the next Papua), and has tens of thousands of relatively inexpensive, often hard to find stamps that offer to you the philatelic chase that so many collectors find satisfying.