I agree either could work , there has been loads of interest so really what it needs rather than discussing Categories etc.
1, A leader or group to bring the idea to actually happen
2, A venue
3, A date that does not clash with regional or national dates.
It's also worth mentioning that racing on bridleways is illegal. Racing can be allowed on footpaths and byways with the necessary permissions and authorisations in place.
Not sure what the situation would be at trail centres but I assume that consent from the landowner would also be required.
There are other private venues which could be worth investigating.
It's this legislation that saw the end of local XC racing and the rise of the Enduro event, which isn't categorised as racing.
There's a bit more info here: --> http://imba.org.uk/public-rights-of-way ... ridleways/
Not wishing to rain on anyone's parade here, but if it is to happen under the Retrobike banner then it'll have to be done properly.