Trem seams to be dying before its born.. Sure when the Q3 source is released some of the community will return, but not as much as a new-er engine. And since youve ruled out porting where will this go? Still birth?
It's not a question of ruling it out, it's a case of practical impossibility. It's taken fuck knows how long to get Trem to where it is on the Q3 engine. With anything nextgen the content times are huge due to the higher poly models required, not to mention the multiple texture maps. The game source alone for Doom 3 is roughly the same size as the ENTIRE Q3 source. It's just not feasible in any sort of sensible time scale.
"Porting" to another engine is really a misnomer. Porting implies that the set of changes required to have something run in one environment versus another is minimal. In porting from e.g. Q3 to D3 the only thing that can really reasonably be ported are the texture maps, sounds and potentially maps. Models are all too low poly and the code would need a 90% rewrite.
For nextgen game engines such as D3 (HL2 doesn't count, it's not really nextgen) I fully expect the mod scene to be fairly sparse, as the time committment required to have any sort of high quality content is getting to the stage where amateurs are going to struggle.
I'd love to be proved wrong, but unfortunately I don't think I will be.
