My theme, if it has wings and/or it flies and I find it intersting. If it has wings and/or it flies I find it interesting! I even built a couple of models of birds, you know like the robin, cardinal, and blue jay! Don't laugh, I'm seroius.
As the eyes have deteriorated I have moved away from 1/72nd scale to 1/48th scale. I like any aircraft that has interesting looks, and can be found in interesting color scheems, e.g. the Thunderbirds, or the aircraft of the "Golden Age of Aviation" .
With certain aircraft I do not care what scale it is I have to have it. The P-51, A-4 Skyhawk, F-86 Sabre, F-104Starfighter, and a few others are examples.
Over the years I have known modelers that are very rigid in their themes, such as pre-1930 aircraft, or WWII German aircraft, etc. I could never do that.
If I have any theme at all it is to build aircraft that relate to a specific event and a specific pilot, for example John Glenns F-86, Dick Bongs P-38 "Marge", Pappy Boyingtons Corsair, Col. Olds' F-4 Phantom.
In my opinion the biggest hole in the model aircraft available is the lack of decent models of the civilian aviation pioneers and record setters, eg. Wiley post's Lockheed Vega (Lindberg, I know), Amelia Earhart's Lockheed Vega and the Lockheed Electra model 10 (Williams Bros, I know), Jackie Cochrans P-36 or P-51(I know, Iknow) in a decent scale such as 1/48th or better yet 1/32, and done in todays modern mold standards. Now that could be an interesting "Theme".
From my point of view, if it were to get too serious it would not be fun anymore. If I wanted to get that structured I'd become a philatilist or a numistmatist. Screw that! Do whatever is fun for you. Oh yeah, If I ever build a piece of armour, shovel dirt over me please!!! (Down boys, I'm just kidding) Cheers, rangerj