I just wanted to propse the parameter usage - but then I discovered that you did nearly just what I wanted to propose
. Personally I'd make defaults such that it works w/o base set replacements in OpenTTD, that is w/o tunnels and bridges. But I guess that's subject to personal preference, I come from a MP server, and there such things are necessary in a heterogenous environment.
If you want more variety and are bored after a certain date, here a collection of ideas, whether sensible or not I leave each one to judge: It's of course feasable via varaction2 to change the look of the tracks after a certain date
Or look of depots, and fences with time. Do you have snowy maglev already? There are 2 random bits (=4 choices) for each tile. Maybe use that for some variation, if appropriate. Or make a version where you abuse the catanery: maglev in a semi-transparent tube, suitable for inner-city transport maybe...
One last plea: Please give you NewGRF a good license so that it won't get lost to the community no matter what, I've seen too many of those: choose GPL or CC-BY (and no NC or ND clause as it will result in your work not being able to be continued by others if you're not around). Both GPL and CC-BY require that you're always credited, whether people ask you in person or not). You're very welcome to host your newgrf's repository on the DevZone
. Send forum mail or join #openttdcoop.devzone on irc.oftc.net for details.