Emoji nametags involve naming an item with a text-based emoticon or smiley intended to resemble the item itself.While older examples exist, it was popularized by Relk Cehi with '<|:)', and was bandwagoned to notable success by other users for a short time. The trend was reignited on the 7th of April 2016, once again by Relk_Cehi, with the submission of a GRU name with a dashing Lenny.
Emoji nametags can be considered SAFE due to the large room for variation and the lack of vested hatred often exhibited towards other running jokes.
Creating an emoji tag relies largely on execution. Go for items that would be difficult to replicate and utilize uncommon characters in order to make your tag stand out. Basing your tag around common smileys such as :) or :| may come across as derivative, so try and be ingenious.
Remember, emoji nametags are not limited to faces; items like weapons can and have been used in this fashion before. Use their uncommonness to your advantage. Don't use the '︻デ═一' (sniper rifle) emoji though, because that one's been done to death all over the Internet.
Lenny faces are also applicable (for better or for worse), and if nothing else can give your submission some dank meme cred. These are best used as a supplement, e.g. if you're putting in a face that doesn't happen to require a particular expression Just don't repeat the face over and over in the same submission. You'll look like a twat.
Note that you may want to take into account the font appearance: item names use the TF2 Build font and descriptions use the TF2 Secondary font. Neither of these fonts are especially versatile in terms of the characters they can display. However, the site defaults incompatible characters to Times New Roman, so you don't need to worry about the dreaded Unicode squares; at the most, it may adversely affect the appearance of the smiley, or result in a mishmash of the two different fonts.