I think the problem with 'Target' is, that some people tend to put in the funny 100x expecting the return any time soon. While that would be of course very nice, we should be clear, that 100 time 0,001 BTC is 0,1BTC and that is not going to happen, when 0,001 is pretty much the most money people are putting there. (now consider that someone could put 1BTC with 100x return. lol. )

So my 2cents would be to restrict it to max 2x unlimited and anything more would be time restricted like someone already mentioned. Let's say a 3 months. It will either force people to go for 2x which should speed up the reinvestments or people will risk they get out of the game with less like in the other timed games.
Also it might be interesting to put restriction on MAX investment keeping current 100x return and lowering the minimum allowed share to enter. (please note I have no knowledge of transfer fees for all of the supported coins, so lower treshhold might be stupid thing to ask for)