A very quick google yielded some awesome inspiring images. When you say conversions, what are your limits?
No money?
upto..... £10?
Green stuff available?
Personally you could go for anything, as
imo a spawn really is the most randomest thing you could probably sculpt/convert due to its ability to look like anything.
You could....
Build a centipede style spawn by joining torsos you have spare, with possessed arms as its legs or any tyranid parts lying about. You could use cut up sprue to form the bulk of the body and base shape, with GS/putty to sculpt the body piece.
Have a look here:
http://www.dakkadakka.com/dakkaforum/posts/list/360092.page
Its pretty much exactly what your looking for, this dude made 39 (or intended to!). Hes use a huge variety and im pretty sure every spawn is different. He uses combinations of
GS, sprues cut up, kit bashing and sculpting from scratch ( or near enough).