I think that sooner or later, OpenTyrian will must be remastered graphicaly, and its good point to made discussion about it sooner to plan everything, and also for example data file format if it will be changed in future it also should depend on graphic remastering plans.
First, a-must-have should be OpenGL backend to hve all kinds of grapical effects like lighting, shadows, maybe even clouds and water, Anti-aliasing.
Object (ships, buildings) graphics. That will be tricky. There are many ways to remaster them. - First, existing models can be converted to SVG or some other vector format. With vectors, it would be easly to resize everything, or modify in future (adding more details), but i dont think that would be good solution. - Drawing new hi-res models based on orginal. That would be also not so good idea, this models would be hard to modify, and below i propose better solutions. - Do remastering "OpenTTD"-way. Model all ships in 3D, and from 3d models make 2d sprites. All this ways of remastering graphics should also come with at least doubling number of sprites in all animations. - Last, best way, but it also comes with most coding work is to have 3d models. Its nicest. looking way, we have very good looking graphics, every model can be lighted, we dont have any sprites limit. But 3d models MUST be totaly "tyrian-style", because i think nobody wants to have bleeding-edge shmup without tyrian look-and-feel and atmosphere.
I think having native SVG (or most likely a proper SVG subset) rendering would probably be the most promising way instead. Super Street Fighter II Turbo HD remix seemed to have been received rather nicely. Put this togheter with MegaMan9 (even emulated bugs)... personally I believe that the 3D hype is finally going to end. Vector art, when produced by good artists, just look great, and I think it would be something not tried for a while.
All the other things could probably be more invasive to the look and feel.
Personally i think that 3d graphics should be ultimate goal for OpenTyrian. And i dont talk about 3d presentation, but only 3d models which should be exact copies of existing sprites, without any modifications to gameplay. Simply, Tyrian contains too many different weather effects, explosions, fireballs, and other colorful, glowing stuff, to not make use from opengl dynamic lighting.