I don't think there's any way for Syphon Filter to return, really. It was always a "not quite there" 3rd person shooter with some stealth elements. Budgets on those kind of games have ballooned to ridiculous proportions. If Syphon Filter returned, it would either need a comparable budget to match Splinter Cell/Metal Gear Solid or go cheaper than that and look like a rubbish alternative. Either way, I can't particularly see it happening.
1. Ape Escape - why Japan Studio are making Knack and Puppeteer instead of Ape Escape 4 is beyond me.
2. Jak & Daxter - Sly got a second chance; Ratchet never left us; why is Jak not still here?
3. Wild ARMs - never played Legend of Dragoon so this is my JRPG pick. Series never had one bad game; Media.Vision are still going, so bring it back!
4. WipEout - I can never have too much WipEout.
5. New IP - wouldn't want them scrambling around only playing on my nostalgia, though. Give me new stuff.