There are definitely many qualifiers as to the substance that can be used as an offering for the consecration. Mainly, it has to be unleavened wheat bread and pure wine, as dictated by these passages from the GIRM and Canon Law:
*Canon 924 ß1 The most holy Sacrifice of the Eucharist must be celebrated in bread, and in wine to which a small quantity of water is to be added. ß2 The bread must be wheaten only, and recently made, so that there is no danger of corruption. ß3 The wine must be natural, made from grapes of the vine, and not corrupt.
Canon 926 In the Eucharistic celebration, in accordance with the ancient tradition of the Latin Church, the priest is to use unleavened bread wherever he celebrates Mass.
GIRM 282. According to the tradition of the entire Church, the bread must be made from wheat; according to the tradition of the Latin Church, it must be unleavened.
GIRM 283. The nature of the sign demands that the material for the eucharistic celebration truly have the appearance of food. Accordingly, even though unleavened and baked in the traditional shape, the eucharistic bread should be made in such a way that in a Mass with a congregation the priest is able actually to break the host into parts and distribute them to at least some of the faithful. (When, however, the number of communicants is large or other pastoral needs require it, small hosts are in no way ruled out.) The action of the breaking of the bread, the simple term for the Eucharist in apostolic times, will more clearly bring out the force and meaning of the sign of the unity of all in the one bread and of their charity, since the one bread is being distributed among the members of one family.
GIRM 284. The wine for the Eucharist must be from the fruit of the vine (see Lk 22:18), natural, and pure, that is not mixed with any foreign substance.
GIRM 285. Care must be taken to ensure that the elements are kept in good condition: that the wine does not turn to vinegar or the bread spoil or become too hard to be broken easily. *
ourladyswarriors.org/articles/badliturgy.htm
So no, a priest can’t just use normal bread or other food product; using an illicit species is a significant liturgical abuse and would invalidate the consecration.
Also, it’s important to note that we don’t want to simply use any form of unleavened wheat bread; we want to use something that will create minimal or no crumbs when broken. The reason for this, or course, is that Jesus is present in all of the consecrated matter including these crumbs. We want to make a strong effort to minimize these so that there are not crumbs on the floor and we end up literaly trampling Jesus underfoot. That’s why the current hosts are in the form they are. Host manufacturers of high quality will go out of their way to mention that they use special edge-sealing processes to minimize crumbs for this reason.
Thanks for the sincerity of your question also.