I am not sure what can or cannot make it to the PSN as some of the great title I can think of were originally for non-sony systems and some of the makers have since gone out of business.
Here are some suggestions though.
A launch title for the Super Nintendo. The Super NES version was IMO the best side scroll space ship fighting game ever and also had the best soundtrack of any space fighter ever as well (look up specifically the SNES version on youtube if you think I'm joking) Researching it reveals that I am not alone.
This game is rediculously expensive right now. IMO, it was the only really good Persona game and I have recently heard allot of old-school gamers glorify it in front of younger ones. Since there seams to be a real fan base for the series now, why not unleash the original on the PSN. I'd buy it. I bet allot of others would too. Not only is this IMO the one that deserves to be on the PSN most, It'll probably be the most successful on my list in terms of gain porportionate to investment.
Mega Man X
If Capcom doesn't mind releasing the NES Mega Man games, why not release the Super NES ones too. Mega Man X was not only a great game but had a very good soundtrack too.
If I could choose one castlevania game for the PSN, I'd choose one of the classics that's already available. But it's not the only good one.
Believe it or not, this series still has a fan base. I doubt it'll be a BIG hit, but it'll definitely sell enough to be worth while. Don't ask allot for it. People are willing to buy it but not spend piles of cash on it. I'd sell the original trilogy together for $14.99 or a remastered trilogy for $29.99 (or $12.99 pre game. My guess is that a remastered version will do better and be more practical. Just a guuess, but I've so far been very good at accurately predicting how things will do in the market.
At one point, Toshiden took the world by storm and was one of the most popular fighting game series around. Then it suddenly disappeared as quickly as it appeared. If I remember correctly, the publisher went out of business. Still, I bet someone can aquire the rights to the pre-Wii titles and sell them on the PSN. Again, I'd buy them.
Soul Calibur 3
Commonly regarded as the best game in the series, the only thing it's lacking is graphics and character creation features. If I were Namaco, I'd put a remastered version on the PSN with an up-to-date character creation system. Not only would it sell anew but you'd have the oppertunity to sell more DLC for it as well. Fighting games are loosing their edge, but with that said, this would probably be the fighting game genre's equivilant to Square remaking Final Fantasy 7 with Star Ocean 4 graphics and expanded features.
Because it's a fighting game, it wouldn't be a huge hit, but it'd probably do just as good as any brand new fighting game.
Resident Evil 1 and 2 remakes.
I know they were originally released only for the Game Cube, so I am not sure what the license stuff is all about, but if possible, I think these would do great on the PSN. They'd probably kill the market for the true originals though. I'd sell them as bundles (new and old version together)
Dark Cloud 1 and 2
People still like them.
Shadow Hearts trilogy
The most underrated JRPG series of all time IMO and in the opinions of the vast majority of people I've met who played it. It's been getting allot of great word of mouth advertising over the years. Put it on the PSN and it'll sell.