First of all ... do you have enough potential block makers in your congregation?? Otherwise, this might not work, in extending the project to involve others.
Then, if you are going to ask others to make the blocks, you need to be prepared to give them the freedom to make what they want and put some trust and faith in what will be made. And to be ready to accept any and all, regardless of the quality that comes.
My suggestion would be to pick a colour scheme, and then select 2 or 3 fabrics to give to each of person. This will allow for some unity, and not get looking to much of a mish-mash. Make up "kits" in ziplock bags with the fabric and be sure to include very clear instructions (oh dear, rules
) but keep those guidelines to a minimum to encourage their own artistic freedom! Eg. size of the unfinished block, all fabrics provided must be used (but not necessarily all of that fabric), deadline to submit, max # of other fabrics which can be added.
You may or may not want to ask them to make a block that has significance to their church involvement and/or relevant to the church's history. Be careful with this concept, as it may prove to be a very limiting factor and you may find several will not participate because of the specificity of it. Again, be sure this is included in the guidelines that you distribute. Don't expect them to remember what you verbally say!
Something to consider, is to charge $ for the block kits. I know in a Church, this may not seem appropriate. However, it may encourage people to finish their block, and return it. It is all to easy to set it aside and later say, oh I forgot or I didn't have time. Sad to say, I know some who have taken the free kits, only to look at it as a way to score some fabric for free! Another reason to charge. In essence, you are only asking to spread the cost of the project among others. Or perhaps there would be some sort of a perk that could be given back to them, when they submit their completed block, to equal their investment (or better!).
Another element to consider ... encouraging the youth to participate. This might be something that could be worked into a Sunday School project or a special Saturday craft get together to help them make their blocks. After all, they are the ones who will be around for the 250th anniversary, and will have that to look forward to!
As for your end design, IMHO it would be a good idea to make some plans in advance. However, you might find that will change when you see how many the blocks are submitted and what they are like. So while you might have plans, and fabric ready, just be prepared with flexibility for the end result.
As a totally different spin to this project ... if you want to turn it into a fundraiser, you could always go back to what often was done in churches years ago, where people paid $ to have their name included on a quilt and then then the names were embroidered on the quilt. One of the nicest ones I have seen was owned by the Church I used to go to (I moved!) ... they marked the outline of a five point star on each block, and then the names were embroidered along each of the outside edges, so ten names per block. I don't know for sure, but I suspect that each person was given a block (or more) and asked to sell the space, and embroider the names. That is all the quilt was ... the blocks and names, and my, it was quite lovely.
Once the quilt is assembled, you might want to set up the quilting frame at the church, and encourage everyone to put a stitch or more into it! This could turn into a whole new group of quilters, that have never had the opportunity!
Whatever you do, just make sure it becomes a FUN project for the Church family ... and not become one they detest!