Speaking as someone working on matrix and riot, we would love you to come and do BetterSlack style things to Riot - we are getting tantalisingly close to being “as good as Slack” (except entirely FOSS, decentralised, standards based and with E2E encryption) - and positively encourage folks to customise the apps as long as they don’t introduce bugs or harm the security model. Plus we can just accept changes straight into the app rather than injecting via an extension. Pleeease come help us make it better - #riot-dev:matrix.org would be the place to come :) And yes, we have bridging to slack too.

I'll be honest, I don't have a huge amount of experience using it, never to a company slack sort of thing and there may be some company policy about this sort of thing (security and all that) so I'd encourage checking there but I hope it works out!