There are many, many films that would work if a sequel spawned. However sometimes it's best to leave a film as a standalone, else the magic can get lost for a second time, or that second time becomes a shameless cash grab or even evolve into a franchise that loses its feet along the way (Die Hard, sadly)
However, off the top of my head, here are a few films that could work with a second film. Some more so than others but I'd be very chuffed to see further continuations of these movies below. Or they may not work for a second time, let me know in the comments what you think.
1. Dredd 3D - More than three years later and this is still one of my favourite films. This 2012 remake finds Karl Urban totally own the red police armour as gruff law enforcer Judge Dredd, who finds himself wound up in a lock downed tower block ran by violent criminal Ma-Ma played by the incredible Lena Headey. Despite great reviews, the film barely made back its budget. One can dream a sequel will make its way onto screens in the future but it's unlikely.
2. Salt - Angelina Jolie needs to come back as Evelyn Salt! I found this to be a thoroughly enjoyable action thriller with a solid story and fantastic action sequences. Spoiler alert, the film ended with Salt set free in order for her to take down further hidden moles within the U.S. I would pay good money to see Jolie return to kickass, and continue the adventures of the Russian sleeper spy.
3. Shoot 'Em Up - Clive Owen knocked it out the park as Mr. Smith, a carrot-munching loner/firearms expert. Ridiculously over the top and very funny, to see Smith return to face another host of baddies would be great.
4. Romy & Michele's High School Reunion - Lisa Kudrow and Mira Sorvino make a brilliant comedy duo in this 90's flick. By the end of the film these two have successfully launched a fashion boutique. A sequel, though unlikely, would be more than welcome, a story possibly concerning the future and finance of their store, and how these two sweet yet bumbling girls would face this. I don't know how it would work but they made a terrific pairing which really sold the first film to me!
5. Final Destination 6 - The fifth film proved there was still life, ahem, or death, in the franchise. We're long overdue a sequel, and as long as the deaths are inventive and brutal, with the cast ready to bite the dust, and possibly a neat twist thrown in to keep things fresh, it should be another great entry to this gory franchise.
6. Clash/Wrath Of The Titans - Contrary to most opinions, I loved these two films. Action packed, epic at times and filled with awesome movie stars, what a way to start a franchise. Where's the third film though? Revenge Of The Titans? Aftermath Of The Titans? Whatever they decide to call it, rhyming title or not, Sam Worthington needs to take on the next set of Gods and monsters.
7. The Crazies - This 2010 remake was superb. Scary, beautifully shot, very well acted and smart too, it ended with the suggestion that the outbreak was far from over, with an aerial shot stating that another city is about to be checked for contamination. More crazies please, with Radha Mitchell and Timothy Olyphant returning!
8. Hansel & Gretel Witch Hunters - Another favourite! Jeremy Renner and Gemma Arterton tearing witches apart in gloriously gory style never gets old. Critically bashed but a hit at the box office, it's a wonder why a sequel hasn't come out yet, but with the busy schedules of its two stars time isn't on the studios side. The film ended with the possibility of further adventures, and that is what I really want to see, more crazy weapons, violence and witch gore!
9. The Equalizer - Denzel Washington has the knack for picking top movie roles. The Equalizer is no exception, as Robert McCall, Washington dishes out the violence as he brings down a notorious crime and prostitution ring. The body count is high, the performances are intense, the action thrilling. Again, here is another movie that ends with the possibility of more stories to tell, with McCall being such an interesting character in a city that has a seedy underbelly.
10. Drive - Ryan Gosling played it smooth, mysterious and effortlessly as the driver, a stuntman moonlighting as a getaway driver. Such a stylish film in every aspect, to see something similar would be great, but a sequel with Nicolas Winding Refn in the directors chair and Gosling would be awesome.