Good topic. As a scientist I like to break topics down and so right away I see 2 areas for discussion. One is "plain versus fancy" and the other is perhaps "sticking to specific historical schools of building versus working in your own style".
Plain versus fancy: This could also involve "frugal versus expensive" but need not. It's possible to build an expensive gun (custom barrel, best lock, best components all around, expensive wood) that is simply styled or even be in the "poor boy" or "schimmel" mode. For example a plain peiece of English walnut would not be fancy, but could cost more than a super piece of curly maple. A plain, expensive gun could be the result. It is also possible to build a fancy gun without using all the most expensive components. An example might be Jim Kibler's use of plain maple blanks to build rifles with elegant design and carving. Or one could use a straight barrel, plain wood, inexpensive lock, etc but have elegant carving and engraving. It wouldn't cost more to build; just more effort.
Ok, enought theorizing. How do I decide whether to build fancy or plain? I generally get excited by an original and that is where my building starts. Sometimes I want to expand my skill set and challenge myself by choosing a design that requires me to improve my carving or engraving skills, or perhaps something requiring more inletting skills, like a Bucks County entry thimble witha big "skirt" or a buttplate with a long and complicated front extension that must be inletted "straight down". Other times, a simple early multi-purpose musket without a buttplate might capture my imagination. All that's needed is a big lock, a long barrel and a plain piece of wood, and the rest of the parts can be made from sheet metal.
When it comes to working in styles of originals or making guns of my own design, I confess, I don't have my own design. My mind doesn't work that way. Some do have that creativity and I admire it. It's probably accurate to say I don't even try very hard to be creative, but I could if I had the desire. I do like to make guns that folks recognize as being in such and such a style. I guess the historical aspect is what drew me into the craft and sport and I still work in that vein.
If I was in a situation where I just needed a gun, and had to work with whatever parts and wood I had available, I'd still probably find a way to make something that folks would recognize as belonging to a particular time and place.
On my "to build" or "in progress" list:
1) In progress, RCA #19, the early walnut stocked "volute" rifle. Modest carving, but requires more engraving than I am comfortable doing.
2) Hudson Valley fowler, 54" barrel, TRS Dutch lock castings will be a challenge as will Dutchy carving designs and the architecture. This will have the two-toned stock with the raised carving lighter in tone.
3) Hickory stocked Bucks County rifle, plain and sleek. Big challenge here is killer hard wood and relying soleley on architecture, no decoration.
4) A near copy of an original rifle probably made quickly and plainly for the Revolutionary War. The buttstock has a big portion that is spliced on, probably due to a major check or break and the one I build will probably have that too.
5) Carolina gun painted with designs like Jack Brooks did. Probably stocked in beech.
6) Short barreled plain early Pennsylvania rifle patterned after one likely made by Berlin. It's in RCA, volume 2 but I forget the #.
7) I need to make a Reading gun and have not yet decided on the design.
8. Big plain early Lehigh rifle based on one Shumway illustrated in Muzzle Blasts in the 1980's. Sideplate and guard resemble Oerter's work but the underside of the buttstock is one smooth curve.
9) Plain musket dating to mid-1700's.
10) Early Dutch trade gun that could have been made 1680-1720.
That's my top 10 at the moment.