Interesting thread! If storage space isn't an issue, it may still be possible to score an old sport-tourer or name brand rigid MTB for cheap and play with it to see what you like, and get something different later on. Of course, I'm spoiled with sport-tourer pricing: I have one I don't ride these days (needs dropout repair) I got for $15 in ridable condition at a Goodwill ages ago, and another I found in the trash (not ridable) and have added about $100 in parts I like. Well, soon to be $170 ....
Interestingly, Mrs. 1987cp has been doing something similar to what was suggested above .... after thinking it was impossible not to hurt while riding, she picked up a brand new comfort bike several years ago and rode it a good bit while thinking how it could be better. Now that our 12 year old has "stolen" it, she's picked up a couple of other bikes, a rare(ish) recent model bought complete and arriving soon, and an '85ish frame we built up with parts on hand!
Right now there's an XO-4 frame on eBay for an okay price, and also a two year old 53cm Rivendell Atlantis (super hardcore allrounder) frame closing in 3 hours at the time of this posting, currently at half retail price! I'm on the lookout for something all-roundish myself right now, which is how I came across them.
Of course, what I need more than another frame is a really large budget and an even larger shed so I can collect and/or horde at will ....