First build I started was a Pilgrim Sumo (there, I said it in public....). I was naive back then.... Anyway...it was, chassis wise, a POS. I made some adaptions to give it coilovers rather than just cut down Granada springs, put a V8 in it, tried to make a half-decent job of the whole thing. Had very little support from the 'factory'. The manual had lots of pretty pictures but little information. Part way through that build I put it on hold for a bit and built our Westy. Back then ('91), there was nothing like this forum so I was on my own. Personally, quite enjoyed the experience of having something that went together properly. One or two issues with missing parts, chrome plating coming off wishbones, stuff like that. Overall, it was a joy to build and has been so ever since.
Next was a rebuild of a previously enjoyed Marcos Mantula Spyder. That was another fine example of why one should never assume a 'manufacturer' has the first clue about what they are doing. Front and rear suspension geometry on those cars was utterly wrong and hard to put right. I re-engineered the font suspension with new wishbones, uprights and rack, ripped out the live axle at the back and put in a cut-down Jaguar IRS. Stiffened/strengthened the chassis while I was at it. At the time I was doing that, it seemed that only one or two people in the 'community' understood what bumpsteer was and that our cars suffered it quite as badly as they did. I did an article for the newsletter about it at the time and for a while I was treated like some kind of leper for suggesting such a thing. A lot of owners weren't very hands-on or technical guess.
Last build was a basket case pile of parts. Most of a chassis for a Stratos replica that had changed hands a few times and then had bits cut off it to try and make it into a suitable basis for an F40 replica. It came with some of the body parts, all in shocking condition. Shortly after I bought it, the company making them ceased trading. That was something of a challenge. Through lack of funds mostly, but also having to make a hell of a lot of stuff myself, it took me 6 years to finish that one.
My next build is going to be along the LoCost route. A real departure for me - previously I have tended to spend what it took, within limits, to try and make a decent job of things. This time round I simply won't have the funds to do that, so this one is going to be a different sort of challenge - make it as cheaply as I possibly can. Use as much from the donors as I can. Totally DIY chassis and bodywork (such as it is). Doing this and not ending up with something that looks like a POS will be tough for me I suspect! It's this by the way www.midlana.com
So, if you want a real challenge, try something previously enjoyed that you can't get parts for, a radical departure form the tried and tested build options as suggested, or totally DIY.