Here in Area 1, for years most recognized events hosted dressage and stadium on grass. It has only been in the last ten or so years that I’ve seen a trend towards competitors pushing organizers for venues with a dressage and SJ ring.
Grass should be a non-issue on all three phases, but I do hear from time to time people complaining at various venues about how they didn’t do well in dressage or SJ “because it was on grass”.
Have you heard from your community that they are unhappy with SJ and dressage on grass?
In that regard, I’m a real hard-ass and have zero empathy with this crowd. I did dressage on the grass at Stoneleigh in the fall… 'Nuff said. (for those that aren’t familiar with Stoneleigh: it was an A1 event that ran spring, summer, fall. The courses were always great for the level and it went over some good terrain and woods. In NE in the fall, it would get muddy especially because a huge portion of the XC was backed up to a major interstate and when it rained the water runoff would leach into the field. Competitors wouldn’t go to it because they didn’t want to “slog through mud” on all three phases – aaaand, we lost the venue because of these competitors… So… Yeah. ).
I would not change any thing your first show. I would use your first show as a gauge to measure the pulse of your community and figure out if Modified is needed in your area/market. Some FB posts or email blasts would help gauge rider responses as well – but I think that social media makes everyone say they are interested and/or going and then when the day comes there are crickets.
I agree with Findthedistance – try to find your niche crowd. Modified would be a welcome addition here in A1, but in NoVA and MD and even NC and SC there are so many modified options that I think adding another purely for the attendance would not draw the crowd one would hope. IIRC, you are out west, right?