Esperanto is interesting but imo you'd have better luck taking a big language in the same group and making a simpler version of it for the rest of the group that almost everyone can understand. So for Slavic, take Russian and simplify it to the point that everyone can understand it and use it to learn other Slavic langauges and speak 'Simplified Russian' with little effort. The Soviets did do this a little but not entirely, though I think they were limited by how much of a statistical analysis of language they could do at the time. If they were still around... :thonk-cri:
Esperanto is interesting but imo you'd have better luck taking a big language in the same group and making a simpler version of it for the rest of the group that almost everyone can understand. So for Slavic, take Russian and simplify it to the point that everyone can understand it and use it to learn other Slavic langauges and speak 'Simplified Russian' with little effort. The Soviets did do this a little but not entirely, though I think they were limited by how much of a statistical analysis of language they could do at the time. If they were still around... :thonk-cri: