Woah there, Sonic, not so fast. Your developments, while very interesting, seem to betray your age bracket. No offence, of course, everyone deserves an equal say, and I was in that age bracket untill a few years ago. In fact, your ideas seem very similar to mine from about 5 years back (heck, it may be even longer now!). So, lets take a quick look at them, because some of your ideas are a bit far-fetched, but I see good ideas too...
4-6 prop engine Shorthaul 400-500 Seater
Sorry, but the market just does not exist for an aircraft in its own right. Even today, the 747 is used domestically only in Japan, and to a lesser extent the US due to high capacities, and in large countries due to its range. Even though jets are expensive, a new aircraft unrelated to anything else would be significantly more expensive to develop, and hence to buy, especially due to the limited market. It is best to stick with derivatives of existing models, which will have attractive sale prices, and are fuel efficient in their own right.
Twinjet longhaul 120-170 Seater
Yes, a long range 737 or A32X makes an interesting proposal. In fact, I beleive the market could be there for this, although it could be expensive (as someone has already mentioned) due to the low pax numbers. Aircraft of these capacities are usually used for short routes, and as such have high passanger-hour numbers (multiply the number of passangers flown daily, by the number of hours flown daily). Larger long haul aircraft have large pax-hour numbers due to the large ammount of people they have. Thus, there would be a question of profitabilty on a route that does not have capacity to justify a 767 sized aircraft over long range. However, it is a possibility.
4/5 jet Ultra large longhaul
First of all, I doubt you'll see 5 engines. 4 engines would be easer to install and service, and would only have to be 1/4 as powerful again as what would be used if you had 5. Basically, the smaller end you have described is the A3XX. Today, it is unlikely there is a market for an 800 seater. However, give it 30 years, and I immagine that there will indeed be an 800 seater. I know Aerospatiale a few years back were looking into an 800 seat 3 or 4 engine flying wing, as were McDonnellDouglas. I would not be surprised to see this in 20 years time!
Trijet 400-600 seater
No, this will remain with the quads (or the twins, if 777 sales dont give out to the A340-500 and -600). There is simply no need to complicate things by going with 3 engines. Tail mounted engines are harder to service and replace. Boing was considering to fit a third engine in the 777ER, of less thrust than the other two, and mainly to avoid ETOPS. However, this didnt come to anything...
4 Prop 200-300 Seater
Props are no good for long range, because of speed, so forget that part. In the shorthaul market, other than the exceptions mentined before, most domestic flights are in the 100-150 seat range anyway. Whether props would offer cost advantages over jets in this market is arguable.
You have a lage seat range there, so I immagine you are thinking along the lines of the Airbus widebody fuselage. This would make for an interesting SST. Of course, you would start out by offering only on to the market, then later on offering streches and shortning as the market dictates. however, Boeing stopped its research into SSTs a few years ago, after concluding there was no real market. It is now thought that perhaps the future of SSTs, for now anyway, lies in SS Business Jets. That being said, it would be nice to see a widebody SST that brought Supersonic transport to the masses. Check back in 20 years, and who knows!
Well, this has got long, sorry! Sonic, you definitely have an active immagination, so keep it up, don't take my criticisms negatively, cos they are not meant like that. Keep dreaming, cos I reckon you have some good ideas, and with a bit more knowledge of the industry, who knows, you could be onto a goldmine!
"So powerful is the light of unity that it can illuminate the whole earth." - Bahá'u'lláh