I don't know what you mean by "essential" - I guess you want to have more vehicles and so on - and I don't know whether you know that there are lots of additional vehicles already available!
Just see here: http://www.simutrans-forum.de/forum/thread.php?threadid=3131&threadview=0&hilight=&hilightuser=0&page=1 [german]
Or here to download (almost) tons of new vehicles( by myself): http://dl.dropbox.com/u/1709823/96c_Zuege/pak/index.html
Two-Floor Regional trains, Two-Floor Intercities, High-Speed trains for 1930, several coaches and a high speed train that is even faster than the NoSe (and lots more)!
some pictures:
http://www.simutrans-forum.de/forum/attachment.php?attachmentid=11850 - Hermes High-Speed-Train
http://www.simutrans-forum.de/forum/attachment.php?attachmentid=11904 - Hephaistos & Chiron
http://www.simutrans-forum.de/forum/attachment.php?attachmentid=11906 - Themis Regional Train
http://www.simutrans-forum.de/forum/attachment.php?attachmentid=11832 - Solina Metro
http://www.simutrans-forum.de/forum/attachment.php?attachmentid=11780 - Sad Sardine
Feedback is appreciated!