IS mechs need to be designed around one of the following ideas:
- Mobility - be able to get in, get out, quick.
- DPS - be able to do as little facetanking as possible and torso-twist away.
- Survivability - be able to take a hit and keep trucking.
- Pinpoint - be able to let loose a single devastating alpha.
On the good side, you have LOADS more options for customability and different mechs. There really isn't a "bad" mech out there, although there are definitely ones that better fit the meta. Every time I see a mech and think "that thing is totally useless," some pilot out there goes and makes it work somehow.
So take something that looks fun, seems to mount the weapons you like to run, and make it work.