Before one can work out what is considered okay to wear, one needs to be aware of what is incorrect. The famous posek, Rav Pesach Eliyahu Falk zt"l wrote an English sefer called 'Oz Ve-hadar Levusha: Modesty an Adornment for Life' published by Feldheim.
He features a whole chapter that deals with sheitels and the potential issues to watch out for (with source citations to back up his points). In the above link on Google books if you read from page 245 he lists a series of incorrect types of sheitels. Some excerpts are as follows:
Sheitels that are unusual, long or eye-catching
Some sheitels have an unkempt wild look to them. Such sheitels do not comply with the basic requirement of midas hatznius, since a woman wearing such a sheitel will be noticed wherever she goes because of her unusual and disarranged appearance. Also, such sheitels demonstrate a carefree approach to life. This is totally out of character for a tznua who is blessed with Yiras Shomayaim and is constantly careful...Other sheitels have been manufactured or set in elaborate immodest styles are are not fit to be warn in by N'shei Yisroel who are inherently refined and self-effacing.
Some sheitels are long and loose, and lie flowingly over the young woman's shoulders or even hang down her back. There could hardly be a more undignified way of fulfilling this mitzvah than to copy hair styles sported by the common females of the umas ha'olam...Many Gedolei Eretz Yisroel (and of other communities) have ruled that women should not wear long sheitels even when the hair is bound together behind the head...
Conspicuous, lop-sided sheitels
Sheitels that a have a deliberate and excessive difference in style on the two sides, e.g. on the right side the hair hangs forward over the eye and covers almost half the face, whilst on the left side it is brushed back behind the ear, are unrefined. This type of style is made to be highly conspicuous.
A Sheitel so well made that it is barely detectable
...it is totally incorrect and against the will of the Torah for a woman to wear a sheitel that has been manufactured to such perfection that to an onlooker (who does not know that she is married) she appears to be an unmarried girl.
The issur to wear a 'window-sheitel'
...This is the "window-sheitel", which is a sheitel with a hole in it. Through it the woman pulls out a section of her own hair This she mingles with the hair of the sheitel that is in the majority, claiming that although her hair improves the appearance of the sheitel, her hair is annulled in the majority of the synthetic hair (בטול ברוב).
So with all of this in mind, a correct sheitel according to Rav Falk would be:
- Of a shorter, modest length
- Clearly a sheitel (i.e. as opposed to one that looks too real)
- Covers the whole head without a hole or any real hair being worked into it
- and of a normal (possibly symmetrical?) cut to avoid any conspicuous looks.