Maybe Lovage? it is in carrot family, but has celery / parsley smell. If you have trouble finding it, buy a tiny plant and put it in a far corner of the yard, as one plant can get very large.

I think I would try bokchoi

Jicama is great for texture in salads where you'd otherwise use celery. It's mild flavor is pleasant and won't seem odd.

Water chestnuts, the stems of Swiss chard or broccoli, or kohlrabi. (Do not substitute celeriac or celery seed.)

Depends on what you're making. If it's being used for flavor I'm not sure there's much you can do about it, maybe a little broad leaf parsley? If it's the texture you might consider cucumber for crunch, or green onion or fennel or chard stems, but all of those will impart different flavors to what you're making, so you'll just have to use your own best judgement when it comes to making a substitution.

From a Mira poix point of view, the italians leave it out and just use carrot and onion. But, don't forget trinity. in the creole/Cajun side of things they call their mira poix trinity and intead of using celery they use bell pepper. Just a thought.