Always keep your options open. As a club, srt the rules and classes to be run for one season and work from there. Don't fall into the trap like a lot of Committee's do and think just because it didn't work last season, last year or a couple of years ago, that it won't work again. What racers want is always changing and if the club keeps up with the changes they stand a better chance of growing stronger.
Maybe for a starting point, as 4wd mod appears to be the flavour for now, stick with that class and work the other classes to suit what members have and sort the rules for those classes from there. As short course trucks are still being sorted out up here, have a motor limit and trucks to use the SC type rims and tyres. They are a bit pricey to upgrade, but from what I have seen so far, you won't be needing a new set each race day as they have a good wear rate on them, and this also keeps the concept of this type of truck racing for what they were designed for.
If you find interest wanes a bit through the season in a class, that could be a sign to look at making changes to a class or two. All racers want to go fast, and all racers like good competition, and you can have good competition between racers if, for instance, for a season you find a common motor that everyone has or would run for a season. This would encourage those who may not be able to outlay to much to make the jump up a class if they know what motor to buy instead of finding out the new motor they bought is still to slow for the jump up.

Open fun days, fete's and displays are also a good way to gauge what can draw interest, after all, you have to encourage young drivers into the club as this is what will help keep the club growing. As adults we tend to forget the youngsters or want to restrict them racing with us as we are afraid they will get in the way or run into us and break things. Having a class for them can be a good thing for a club if your looking to increase membership.

20 years ago we used to have end of season fun days, racing with no marshals, the car that ran the longest, mini enduro's etc. Most times we had more members turn up for those then race days..., and due to location, we also got a lot of interest as well and picked up racers for next season from those.
Just a few of many things you can consider, it all really depends on what the committee wants to do which will determine how ell the class will go