In my opinion, Under Siege 3 is possible to develop. It doesn't necessarily need to be a Warner Brothers production - it could be co-produced by one or more countries outside the US such as Russia, China, film, and/or others in which there is still a decent market for his output. If it is a good film, I don't think the audience will be bothered by who paid for it or where it was filmed. The main concerns are a good story, decent production values, and enthusiasm by Seagal.
A simple story which plays to the age of the character as well as Seagal's current abilities is all that is needed. Casey Ryback has expatriated to Russia or an Asian country. Working as a part time chef in a hotel, or a volunteer chef at an organisation, either of which set the scene for a hostage situation. That would allow the action to be more tactical rather than physical. Perhaps a notable costar to draw in a broader action film audience, but not a star like Jackie Chan who would completely overshadow it - maybe someone like Statham or Lundgren.