A few things I'd like to see. The first game was pretty good but a tad overrated by a few here. It's still got a ways to go before becoming one of the top shooters. I do think they'll be able to do it and then some with the sequel though.
1. Better visuals, the open areas were very bland but I assume the engine improvments that happened for Ratchet will help fix this. Other things like more variety, more areas with fully interactive awesome looking water, snow, etc. More weather effects like light rain, heavy rain, mist, fog, thunderstorms, blizzards, etc. More in the way of elementals. The overall lighting and shadowing needs to be more pronounced. It needs more color (and no I'm not talking rainbows and flowers), but it just felt too boring and uninteresting in many levels of the first game. Another thing is to have lots of different styled levels in terms of time of day, weather, and location. Later parts of the first game were great looking, but a lot of other areas were really weak visually and artistically.
I'm a water whore, give me lots of gorgeous water! High quality H2O! The blizzards were fucking gorgeous in Resistance, keep that for the second game.
2. Vehicles fully intergrated into gameplay. Same for multiplayer, add them into that, but to please the whiners make them optional with the ability to play games with them turned off. Some will cry like babies about adding them, but they need to be there. The gameplay in multiplayer at times felt too thin, and this will help in adding some depth. This isn't to mimic Halo, Haze, Unreal, or other shooters, the fact is Insomniac could do some great things with them and it'd make for a better game. Don't like them? Then turn them off in multiplayer. Some love them, others don't.
3. More impact and kick from some of the weapons. The Cabine looked kinda weak overall, design and fire wise. It needs a major boost in terms of kick and punch (both audio and visual). Rossmore felt great, just needs a visual upgrade to a more modern look which I hope the second game will have. Really get creative and wild with some of the new weapons. Bullseye was great, Fareye was great.
4. Do not have weapons in single player that are not in multiplayer, and don't take weapons out of the online space. If they're unbalanced, balance them. I love me the HailStorm.
5. Much better story presentation and overall character development is needed. None of the characters were really well presented in the game, and I really don't care about any of them. Hale isn't exactly a good lead character, but who knows, he may have some potential it just needs to be tapped. The way the story is told needs to be changed and presented in a more cinematic way. It has lots of potential. I'd like to see more story told through the first person perspective like Half-Life 2, mixed with gorgeous cinematics.
6. Move away from the WWII era setting. To be honest, I hate it. Move 20, 30 years into the future. Perhaps even having the majority of the human race fully enslaved by the Chimera and the humans having to rise up and fight back to reclaim their land. You could also have pockets of Resistance fighters living out in spooky ghost town type areas and what not. Keep a good balance of huge open battles with vehicles, huge infantry only battles, smaller battles, epic mini boss encounters, close quarters battles, reconish sniper sections, slower paced survival horrish areas, etc.
7. More unlockable character items. Stuff like masks, helmets, etc. Lots of it.
8. More personality and individuality for each multiplayer map. They need to be very distinct and have their own flare. Halo does this well with their maps, where each feels like it's own cool little place.
9. Four Player Online Co-Op. Yeah, this is a must have. Now that I've played it in Halo 3, and the fact Haze is doing it. This needs to become standard. It's just too much fun not to include.
10. I'd like to see the Auger pimped out more in terms of style. Have each round be a small sharp parasitic lifeform that manically laughs and screams as it burrows through the wall. Kinda like the 'Glove Of Doom' from Ratchet mixed with elements of Oddworlds live ammo, with an evil twist.
11. Personal favor. Get Sam Cassell to do the Chimera motion capture. Maybe scan his face in for their designs... Actually that might be too scary, just stick with their normal look.