An alternative would be to write an openSource formula language interpreter, for example with java and an openSource parser generator like ANTLR. Lots of work, but I would like it.
According to Maureen, there will be a formula debugger in an upcoming Eclipse-based version of Designer. It will not be in Designer 8.5, but in a later version.
I used it.... I never relied on it, but I used it. And now, I wish I had it back too.
We have several complex formulas where it would be nice to put a @debug([Wait]) for break points or an @debug([Watch];field1:field2) for a watch list. When either is hit, the formula debugger opens in a similar fashion to the LS debugger and we can watch or walk through the code. One column formula is SO complex, that we cannot add another byte without crashing the client (seriously, I am hoping to optimize this just a bit soon).