A small proof engine, and maybe a limited amount of rules (depending on the exact technical meaning of 'rules') is a good thing, but I'd say a limited amount of syntax is a bad thing. Mathematicians as a community tend to be happy with—I might even say to prefer—lots of syntax, and, if you want to get a formalisation process really going, layering it with enough syntactic sugar to bring in mathematicians who aren't usually 'formalisers' is going to be essential.
It's also true that there are lots of attempts at formalisation out there; I happen to know about vdash.org , for example, but there are certainly others. I think it's good to have an abundance of formalisations, since no one formalisation style is going to appeal to everyone (for example, as already discussed above, probably the more formalisation-minded mathematicians will have a higher tolerance for minimalism).
It's also true that there are lots of attempts at formalisation out there; I happen to know about vdash.org , for example, but there are certainly others. I think it's good to have an abundance of formalisations, since no one formalisation style is going to appeal to everyone (for example, as already discussed above, probably the more formalisation-minded mathematicians will have a higher tolerance for minimalism).